They should buff Eye to Mask of C’Thun’s effect (10 random damage), and likewise buff Mask of C’Thun to deal 12 random damage.
They completely overestimated the value of split damage, thinking too much of Cinderstorm which isn’t good because it’s 2 more damage for 2 more mana vs Arcane Missiles, but because it’s a low mana value that’s easily worked with unlike Eye and Mask.
And before anyone mentions it: yes, I know Mask isn’t part of the C’Thun the Shattered set. I’m just saying if they buff Eye to Mask’s damage output, they should also buff Mask’s damage output (which should happen regardless since it’s far worse than the other “of (old God)” Spells).