Death Vision is one of those spells that has an effect that seems a little outsized to what it costs in energy and time to cast.
I think it's a great information spell, but it's "mentally stunned" with no resistance is pretty easy to abuse. Don't agree? Have a Death Vision-25 foe fight the PCs along with a few allies to take advantage of the -4 defenses from stunning plus the ease of getting a -2 for flanking on a stunned foe.
There are two ways to control that:
Add a resistance roll. Simply put, an unwilling subject can resist. I'd go with Will-2, here, like Madness, because the effect is dangerous but minor, and it's a time-consuming spell to cast until your skill is 20 or 25.
Add immunity. Not everything should be subject to this. Limiting this to sapient or near-sapeint foes (IQ 6+) is a good start. Limiting it to such foes makes it dangerous against intelligent foes, but not an easy way to divert dire animals. I'd also blanket in Death Vision as a spell subject to Immunity to Mind Control Spells - demons, undead, etc. shouldn't generally be subject to this kind of spell.
In my own game, I'm considering doing both. The main intended use - figuring out what might kill you - is unchanged. Its main actual use - stunning foes as an attack spell - would be weakened without being nerfed. The Will-2 resistance means it's still very effective, but it's not automatic. So that's probably how I'll run it in Felltower.