Stray into Shadow is a Spell.
How to Get Edit
Beat the thirteenth mission in Jekk's Bounty, "Waking Nightmares".
A permanent debuff of Health can stop units that come back from the void, such as Dawnwalker.
It's also a board wipe, similar to Harsh Rule that can make your low attack units trade better against any survivors.
The health removed is a permanent debuff that will persist across zones.