The wall of flames is a barrier located around D:25-D:27 in the Caverns of Chaos. It is located either on the up or down staircase. Possessing multiple counts of fire resistance, or even immunity, will not allow the player to pass the barrier; the game will acknowledge them but report that it's still impassable "even for someone like you".
There is only one way to get through: attempting to cross it with the Chaos Orb of Elemental Fire in your inventory, which can only be found at the top of the Tower of Eternal Flames. This will cause the wall to flicker and disappear permanently.