It is often the strike you can make as an opportunity attack, but still is a main action otherwise.
It isnt a free action.
(Emphasis made because those were my realizations when reading the book.)
The Flesh a Crucible - (2d10+2)
(87) + 2 = 17
Conflagration - (2d10+2)
(15) + 2 = 8

Zombie's Clobber and Clutch vs Shrike - (2d10+2)
(26) + 2 = 10
Zombie's Clobber and Clutch vs Obsidian - (2d10+2)
(26) + 2 = 10
Arcane Disruptor - (2D10+2)
(105) + 2 = 17

Skeleton's Bone Shards - (2d10+2)
(84) + 2 = 14
Zombie's Dust - (2d10+2)
(47) + 2 = 13
Melee weapon free strike - (2d10+2)
(41) + 2 = 7


Ghoul's Razor Claws vs Marbin - (2d10+2)
(79) + 2 = 18
Ghoul's Razor Claws vs Vordokov - (2d10+2)
(46) + 2 = 12
Two Shot - (2d10+2+2)
(103) + 4 = 17