Spells: If playing with spells, choose 6 spells, and take two of each of them, and shuffle them together to create the spell deck. At the beginning of your buy phase, place all cards on the spell mat onto the cantrip mat, and all cards on the cantrip mat at the bottom of the spell deck.
I'm not quite sure I follow this. If I place all cards on the spell mat onto the cantrip mat, then all cards on the cantrip mat at the bottom of the spell deck, isn't that the same as putting everything on the spell mat (and anything that was leftover from the cantrip mat) at the bottom of the spell deck? Also, how is the ordering of cards at the bottom of the spell deck determined? I haven't read the cards yet so maybe it will make more sense as I keep reading...
I was going to comment on all the cards, but I'm having trouble following how things interact with the cantrip mat, so I'm only going to comment on some of the cards.
Eternal Lamp - I'm not sure how balanced this is, but from a practical perspective, it's not reasonable to look at 7 cards. I don't think any official card lets you look at more than 4 and make a decision based on all of them. Looking at seven would be very slow. Also, a more minor nitpick, you could have it say "Look at" instead of "Reveal", since the other players don't need to hold you accountable for anything.
Haunted Castle - It should read "When another player plays an attack card, you may reveal this from your hand. If you do, you are unaffected by the attack." The way that you have it worded has several problems: 1. I don't think "being attacked" is defined anywhere in the rules, 2. your version doesn't specify where to reveal the card from, and 3. Your version would block the attack for all players. Furthermore, nothing is defined as the "part" of the card that is the attack, so saying "the attack has no effect" would imply that the card does nothing. (For example, if I play a Militia and you reveal Haunted Castle, I don't even get +$2.) I don't think you intended for any of those things to happen. Otherwise, the card might be fine. It could probably cost $3; the reaction is not much better than Tunnel's reaction.
Tome of Secrets - Where does the Potion come from? Do you mean "You may put a Potion from your hand on top of your deck"? If that's what you mean, then this is a really awkward card. Presumably, you would make a special rule saying that if Tome of Secrets is in the kingdom, include Potions in the supply. But even then, this card is pretty useless whenever there are no other Potion costing cards in the supply. I'm not going to buy a Potion, just so that it might connect with Tome of Secrets and I get to trash a card, while slowing down my next turn. Even if I am getting a Potion for another reason, I'm not sure that it's worth spending $4 for a Tome of Secrets - unless my deck is already thinned out, it's unlikely that my Potion will collide with Tome of Secrets, and if my deck is already thinned out, I don't need to trash anything. I really can't think of a case where I would buy it. You could probably change it to "You may put a card from your hand on top of your deck. If you do, trash a card from your hand." Then it's a cantrip trasher, but it leaves your hand size 2 smaller.
Mirror of Souls - Sorry, but this is a terrible card. First, note that giving it an Attack type doesn't do what you want it to do (unless what you want it to do is just to make it so that Squire can gain it when trashed). If you reveal Mirror of Souls to give me no actions, I can't reveal Moat because you never played an attack card. You revealed an attack card, but you didn't play it. Regardless, I think the card should be scrapped anyway. It completely kills engines, which is very anti-fun. If you have one in hand every turn (by scheme or a thinned deck or whatever), then I am literally forced to play big money. But most of the time you won't reveal one every round, so it's not that bad. If you reveal it sometimes though, it becomes very, very swingy. If you happen to draw it on my double Madman megaturn hand when I'm going for Hermit/MS, I pretty much just throw the game right there. In multi-player, I imagine it gets much worse. I don't think anyone should risk going for an engine in a 4-player game with Mirror of Souls on the board, because if you trash your deck down to a pretty good action density, you'll most likely end up with a lot of dead cards in your deck (since often times the actions won't do anything). Anyway, you can playtest it if you like, but I imagine it will be an incredibly frustrating card, and not fun. If you like it, then I'm not going to stop you from playing with it.
Vorpal Sword - A minor concern is that there's no way to track it. You label it as a Duration, but the thing that it does that happens outside of it's on-play effect causes it to get trashed anyway, so it will never be in play not during the turn you play it. That was a confusing sentence, hopefully it made sense. Anyway, the bigger concern is that again, this card seems anti-fun. I spend a $5 hand to get this card, then it's a Silver until I'm ready to essentially nullify everyone else's turns. With a 2-card random hand, you'll almost certainly not be able to do much with your turn; maybe if you're lucky, you'll get a $3 or even $4 card. But note how much weaker a random 2-card hand is than drawing five, then discard down to 2: This is the same reason Outpost's 3-card hands are so much weaker than Militia's 3-card hands. Maybe it's not so bad as I think about it more, it might be comparable to Pillage (although I'm guessing this is much stronger, because it's a Silver until you're ready to kill it, and then when it strikes, it hurts much, much more). I would say at least let them draw up to 3 cards. Also, this card has the same problem that Mirror of Souls had with the attack labeling. Since you trash it at any time during your turn (not when you play it), players can't block it with Moat or Lighthouse. Also, a much much more minor nitpick: there should be a horizontal line separating the $2 from the rest of the text.
Diviner - First off, you forgot to give it a price. I'm guessing it's supposed to be $5 or $6, probably $6 since it's a terminal gold. Then, the wording is really confusing, I had to read it three times before I figured out what you were trying to say. I'm actually not really sure if there's a better way to word it. If you want to be consistent with wording on official cards, it would be "The player to your left looks at the top card of your deck." I think that's also a little clearer then, that I can't see the card. Anyway, for how it actually plays out. I don't think the discarding option is really necessary; 95% of the time, you would rather either draw it or trash it. The only exception I can think of would be for draw-up-to-X engines, but even then, it's fairly situational. The discard option is probably unnecessary extra text. I'm guessing the card is too strong for $5 (terminal gold with a bonus is probably too much for $5). It might be okay at $6, but I'm guessing you will often times prefer Gold. I would suggest tweaking the vanilla bonus and bumping down the cost, maybe make it +$2 and cost $4. It's actually a pretty interesting card overall.
Dragon - This is way too strong. Also, there's no accountability. That's a more minor problem, so I'll complain about that first. The other players should reveal their hand. Otherwise, there's nothing stopping them from saying "I only have two treasure cards in hand" or "Copper is my highest valued treasure card in hand". (By highest value I assume you mean most expensive? If you mean how much it's worth, then you get problems with Bank.) It should read "Each other player reveals his hand. Each other player with more than two treasure cards in hand trashes the most expensive treasure card from his hand." Regardless, it's way too strong, and very swingy. It's a powerful attack, since it just straight up trashes Gold, and the response to it is to get more Gold, which you do by getting Dragons. It basically means that any $6 hand spent on Gold is a wasted buy, because it could have just been a Dragon, which will get you more Gold, trash other players' good treasures, and can't be trashed by Dragons. It's swingy because if it hits an opponent's hand of all Coppers, they benefit from it. Maybe that's not a big deal though. In fact, I kind of like that the time when it's most likely to hit a hand of Coppers is early on. That means that if you get lucky and get a Dragon early, it might help your opponent. Still not enough to compensate for being way overpowered though. If you get a Dragon early and manage to hit the opponent's Silver, that's probably enough to conclude that they lost that game. It really hurts their current hand, it really hurts their deck, and it sets you really far forward with the top-decked Gold.
Magician - This is interesting. I would change it to "Look through your deck" instead of "Reveal your deck", since the other players don't need to see it. It could be slow, but maybe it's not so bad, probably not too much worse than Scavenger. I was actually thinking about a hypothetical card similar to this yesterday when I was trying to evaluate a different card I had come up with. It's probably okay at $6 - cantrips don't stack well at $6 because it's hard to get lots of them. It might be fine at $5, you'd have to playtest it. I think being slow is really the main concern, other than that I quite like this card.
Sorcerer - Maybe too strong. +3 cards, +$2 is a lot. $7 is pretty expensive though. It might be okay, it's hard to say.
Now for the spells.
Illusion - This is worded horribly. I'm not sure that there's a very clean way to word it. The best I can do is, "Up to one time this turn, when you play an action card, play it as if it were any other card in the supply costing up to $1 more than it. The played card is that card until it leaves play." Kind of copying the Band of Misfits wording, but it's hard to do it when referencing a different card than the one you play. Anyway, it's an interesting card.
Wait, so now that I'm commenting on spell cards, I'm confused as to how they work. Do the cards just say things that are always the case whenever they are on my cantrip mat? Oh, now I understand why it's called a cantrip mat. Okay, I think I kind of have a sense of how this works...Do they only trigger once, during a special spell phase or something? Before your action phase? I'll just assume that that's how it works, because it doesn't seem to be clarified elsewhere (sorry if it is and I missed it).
Fireball - I assume you mean "Each other player..."? Otherwise, it's pretty terrible; it hurts you just as much as it hurts everyone else. If that's what you meant, then it's probably all right. I would worry that it stacks with other discard attacks and with itself, but presumably spells are hard to stack, so it might be okay. (I haven't looked back at the other spell-related cards yet now that I think I understand how the mechanic is supposed to work.)
Summon Elemental - Probably fine, most likely on par with the other spells.
Lightning Storm - Again, I'm assuming you mean "Each other player..." I would re-word it to "Each other player gains a Curse. Each player who did draws a card." That's a little clearer. But actually, I don't think you need the drawing clause. It's probably weaker than the other spells with it. It would probably be fine at "Each other player gains a Curse."
Healing Power - Is there any reason why this says "costing less than $1", instead of "costing $0"? There are no official cards that can cost less than $0. Costing P is neither less than $1 nor is it exactly $0, so that's equivalent in both cases as well. It might be a little weak. You can probably just have it trash any three cards.
Watchfire - It's probably weak. Really the only case where it's better than Summon Elemental is in response to discard attacks. It should probably be draw up to 7. Also, it should be "Draw until you have [6/7] cards in hand."
Antimagic Field - This seems really swingy and frustrating. I don't think there's a good way to fix it; I would just scrap it.
Those are my comments. I hope I didn't come off as too harsh. A few of the cards have major problems, but some of them are interesting. Hopefully my comments will help you to see those things so you can improve. Often times it's easier to see problems with other people's cards than with one's own cards. Good luck with any changes you make.
