Hearthpwn

I'm not convinced that comparing to Eye of C'Thun is a fair example, given that the random damage done by the Eye is only part of the effect.  By the same token, you could compare it to Arcane Missiles and complain that it costs 7 times as much but only does three times the damage.   An1ron's point is well made, in that Mask really comes into its own following a board clear.

Mask is a test like every other powerful card in Hearthstone where the onus is on you, the mage's opponent, to anticipate and work to counteract their threats.   No different to Tickatus, Psychic Scream, Leeroy or a dozen other match-defining cards over the years.