There is so much going on here, it is hard to know where to start.
I want to begin with taking a broad view of what playing one of these piles will look like. The game will basically divide into three sections: (1) the early portion, when players can buy (and subsequently play) the top split-pile card, but are not affected by the Monster's ability; (2) the Monster portion, which starts after the last top card is gained, when players are affected by the Monster; and (3) the late portion, which starts with the player(s) receiving the reward for defeating the Monster, after which the players can buy (and play) the bottom split-pile card, and can continue to play the top card.
I think in designing the cards, it's going to be really important to think about what the game looks like in each of these potions. If getting the Adventurer card makes play slow and cumbersome in the early section, that could slow players from gaining the rest of the Adventurers and moving the game along. It might even cause a player not to gain any Adventurers at all.
If a player doesn't like the Adventurer card, and can design a deck to be relatively unaffected by the Monster, then that player can ignore the mechanic completely. That's not entirely a bad thing, but if it forces the other player (who opened with two Adventurers and therefore is somewhat stuck) to buy the rest of the Adventurers and kill the Monster herself, the reward had better be substantial, or playing those Kingdom cards will become a losing strategy.
Design-wise, they are a double-sided landscape-type card placed within a Kingdom pile.
So, right of the bat, it is worth pointing out that in the official game, there is no such thing as a "landscapes-type cards" because under the official rules, landscapes are explicitly
not "cards" as that word is used. The main effect of this is that cards like Bridge or Highway which makes "cards cost . . . less this turn" do not effect the price of Events or Projects. That's obviously not a problem here, as the only "cost" of the Monsters in in raiders, which are not going to be discounted. However, given the fact that some Monsters (like Goliath and Centaur) affect what "cards" do, you need to clarify if other Monsters (and their rewards) are "cards" (especially if you are not going to make the 1 Monster per Kingdom a rule).
It also seems (for lack of a better phrase) physically strange for a landscape to be in the middle of a pile of portrait-aligned Kingdom cards. How would that even work? Would you rotate the pile 90 degrees when the Monster emerged, then turn it back when it is defeated?
I'm not sure I have a great suggestion about what to do here. The obvious possibility is to rotate the Monsters and make them regularly aligned cards, but I don't think I like that solution. I just think you need to be really clear about what they are and how they work.
The split pile introduces a new resource to collect, Raiders, which are collected (as coin tokens) to a Raider's mat. You commit these Raiders to battle the Monster in the new phase of the game imaginatively called the Raiding Phase, joining forces with the neighboring kingdoms to destroy the opponent. During the Raiding phase, you check whether the Monster has been defeated by counting the Raiders on everyone's mats. If there are enough Raiders across each player's mats, the Monster is defeated and in celebration, it is flipped over to reveal the prize each player receives immediately.
Aside from a provincial distaste for the name "Raiders" I personally hold due to a local sports rivalry (which should have no influence on these cards' design), I do have some possible issues with the Raiding phase. As you state, all that happens during that phase is that "you check whether the Monster has been defeated by counting the Raiders on everyone's mats." Moreover, this does not even happen every turn, as you only check during the portion of the game during which the Monster is active. It is entirely possible that the first time you do this check during the very first active Raiding phase (after the Monster is revealed), you will find there are enough Raiders committed, defeat the Monster, and never check again. To have a phase for something that may well happen only once seems unnecessary. Thus, my inclination is not to have a Raiding phase, at least not if you leave the process of defeating Monsters as it is.
I also have a bit of trouble with the concept of Raiders being "committed." My understanding is that every Raider you gain is committed to battling the monster. Does this mean that if I have 10 Raiders and my opponent (in a two player game) has 6 when I buy the last Hired Hand, that after his next Action phase I would get +10 Cards, and at the end of the turn he would get +6? Even though the Ogre only took 5 Raiders to defeat?
This brings me to a more radical suggestion, which I am not totally certain is the best idea, but something I want to put out there. Rather than just checking whether the total number of Raiders is enough to kill the Monster, once the Monster is active, then during your Raiding phase you actually make the choice of how many Raiders (if any) to commit to the attack, and actually set those aside (maybe on a different part of the mat), possibly with some limitations. Those Raiders are spent when the Monster is killed. This would probably require upping the reward, but could make who kills the Monster less dependent on who happens to buy the last Adventurer, and offers strategic choices about how many Raiders you actually want to commit (and maybe there are limitations to how many you can commit each turn).
Before I get into the individual cards, I want to point out that I think it is going to be EXTREMELY IMPORTANT for you to work out a specific syntax for how these cards work and stick to it. You will need a term for when a Monster is revealed, etc. So, for example, Young Hero says "Gain one (+---) per . . . " whereas Divination Camp as "+ (+---)" With official cards, "gain" usually refers to getting a card, whereas when you get resources represented by tokens (Coffers, Villagers, VP tokens), a "+" is used. Thus, Divination Camp is probably the better bet, but you should fix Young Hero.
Also, most of the cards say "+1(+---)," but Divination Camp and Shipbuilding do not.
Young Hero -- I have real concerns that this card is just too weak to justify getting it early, meaning players will frequently opt out of going for this pile altogether. This is especially true if there is strong terminal drawing on the board. A Smithy-Big Money deck would likely be buying Provinces well before its opponent slew Goliath.
Goliath -- I get the general idea here. If you played a Village, you would get +1 Card +1 Action instead of +1 Card +2 Actions. However, the language is both not in line with what officials cards would say, and (in at least some cases) ambiguous. For example, if one played a Fishing Village, would the -1 Action apply to both the current turn and the "start of next turn" effect? Or would that be -2 Actions?
Centaur is pretty clear cut as it applies to basic draw cards, but how would it work with alterative drawing is much less clear. For example, if you played a Divination Camp, would you still guess and look? And if so, would you not keep the card if you guessed incorrectly? How would Wishing Well work? Would you forgo the initial +1 Card but get to keep the card if you guess right? Could you choose to flip that, keeping the guaranteed card but only looking at the other one? What about with Library? Do you only draw up to 6? What about Scrying Pool.
As to the Reward, was it your intention to penalize the player on whose turn the Centaur is defeated (and, unlike with the Ogre, draws immediately, after the end of their Action phase, (generally) drawing Action cards dead)? A possible alternative would be to give Horses instead of +Cards (which fits the Centaur theme).
It's very late where I am, so I am going to call it for now. I will keep at these, but with a lot of them it might help if you were to go through and explain what your thinking is behind them. That might help the feedback be more productive.