This monster has appeared in a few Final Fantasy games (IV, VII, and XII) and generally takes the form of a trap wall so to speak. It's a little different in each game, but typically has a lot of HP and lethal attacks.
FFIV: This monster attacks after you get the Dark Crystal in the Sealed Cave. He'll close in on you and if you don't kill him fast enough he'll use Crush, which will KO one party member. Has 28,000 HP in the original FFIV and its ports, and has 99,999 HP in the DS version.
FFVII: Was known as Demon's Gate. Doesn't have any attack that'll kill in one hit, but has a good number of powerful attacks. Has 10,000 HP.
FFXII: There are 2 Demon Walls in this game, both in the same dungeon. The first one is stronger and has 22,187 HP but is optional, while the second is weaker and has 10,332 HP but is mandatory. As in FFIV they'll close in and if they get manage to get close enough to the other side of the room (from where they were) it's an automatic game over. (the second one has to travel a longer distance and there are ways of stopping him temporarily) Both can also use Telega to teleport a party member out of the battle. (only cure is to go to another area)