We modify everyone's favorite infinite loop:
Fewer cards I have found to loop infinitely.
6 cards, no Event
Kingdom: Overlord, Lurker, Raze, Watchtower, Crown, Mandarin
Setup: trash Mandarin
Deck: 5 Overlord, 2 Watchtower
You have 2 Overlord and Watchtower in your hand.
play Overlord as Crown
-- play Overload as Crown
---- play Watchtower
---- play Watchtower
-- play Overload as Crown
---- play Overload as Crown
------ play Overload as Crown
-------- play Overlord as Raze (trash itself)
-------- play Overlord as Lurker (gain Mandarin, reveal Watchtower to trash)
------ play Overload as Lurker (gain Overlord)
---- play Overload as Watchtower
Now you have 5 Overlord and Watchtower in your hand.
LOOP FROM HERE
play Overlord as Crown
-- play Overload as Crown
---- play Overload as Crown
------ play Overload as Crown
-------- play Overlord as Raze (trash itself)
-------- play Overlord as Lurker (gain Mandarin, reveal Watchtower to trash)
------ play Overload as Lurker (gain Overlord)
---- play Overload as an Action costing up to $5
-- play Overload as Watchtower
LOOP END
Use Band of Misfits and Ferry instead of Overlord to make this more practical.
We add Library, Village, Trader, Pathfinding to the kingdom. The setup is this:
Trash down deck with Trader to just 7 Villages, 4 Overlords, Watchtower, Trader, Wish, trashing all Silvers in the process. Put +1 card token on Overlord.
Start of turn: 7 Villages in hand. Play all, drawing 2 Overlords, Watchtower, Trader. The rest of the deck is Wish + 2 Overlords. Now we do:
play Overlord (drawing Wish) as Crown
-- play Overload (drawing Overlord) as Crown
---- play Overload (drawing Overlord) as Crown
------ play Overlord as Raze (trash itself)
------ play Overlord as Lurker (gain Mandarin, reveal Watchtower to trash)
-------- topdeck 3 Overlords
---- play Overload (drawing Overlord) as Lurker (gain Overlord, topdeck it)
-- play Overload (drawing Overlord) as Leprechaun (trash Gold with Watchtower, topdeck Wish)