I've got a pile of ideas for its actions but need to sort them out first.
Before that, the current draft does way too much damage to be Challenge 12.
Typically, it'll
Multiattack with either three
Falchion attacks (66 damage) or two falchion attacks plus an Evil Eye (71 damage). If ONE of those falchion attacks hit, which seems likely, it gets to use
Fomorian Butchery for +22 damage both that round and its NEXT round, as the butchery in your draft says "since the start of its previous turn".
Let's see, so if has four or six tries at +12 to hit, if it's facing a target with, say, AC 18 it needs a 6+ to hit for a 75% chance of hitting, meaning the odds of it missing ALL of its falchion blows is at worst 0.39% (for four attacks) and at best 0.0244% (for six attacks).
So the butchery's almost a given, increasing the DPR from 66 or 71 to 88 or 93. Plus even if it doesn't get to use its Reaction for butchery, you've given it an opportunity attack with
Falchion Mastery so it'll probably get to attack with that too.
Plus as the cherry on the top, you've given it the option of making an extra attack with its falchion if it scores a critical from
Fomorian Brutality. With three attacks a round it has a 14.2625% chance of triggering. That adds another 3.13 or so to the DPR for an average damage around 91 to 96.
Putting those numbers in the CR Calculator says its Challenge Rating 14 not 12.
What I'd do is trim down the number of attacks and likely modify or add a few actions to make the combat a bit more interesting. At the moment, the optimal strategy for the Butcher is
Curse of the Evil Eye and then nothing but slash attacks with the occasional
Evil Eye for spice.
That seems a bit monotonous and apart from the bonus attacks and damage type doesn't help distinguish it from a regular Fomorian. Doing slashing damage instead of bludgeoning isn't that memorable a difference.