Great Unclean One makes your opponents lose life each turn with the potential to put tokens into play
Often the attacker can get away with a bit more in the realm of bluffing and offering trades
If all players in the game agree to it, that rule becomes true for this game
Of course, good will alone wont win you the game, so Font of Mythos will only be a good fit in decks that know what to do with the extra cards they draw (or how to punish foes for being flooded with cards)
The graphic they added to the Mystery Booster packs had a square background, and the one they used for The List seems to have a rounded background, which creates a telltale difference in the shape of the cards border as you can see on Chatter of the Squirrel and Ancient Den as well as subtle changes in its placement on new border cards, as seen on Boros Challenger
If you really want to, you can discard two artifact cards