Auto-Reanimation spells on Monsters: How should I CR this?

[Thealtruistorc]

As a DM who plays a lot of video games, I had an idea for a magic item that, when worn by a creature who has just been killed, reanimates it as some form of undead (typically wights or ghouls, but could go all the way up to Mohrgs or Greater Shadows). The concept leads to a lot of interesting scenarios, but I'm curious to know how it would affect the CR of creatures who would die and reanimate mid-fight. I was thinking that the CR would be one less than having both creatures in the fray at the same time, but I would like to consult you guys in this idea. What's a balanced way to utilize this?

[dascarletm]

I would just treat it as two separate encounters (assuming one minion).

If you have more, that's a bit more tricky.

[Deadline]

For reference, there's already a creature that does this by the rules, the Deadborn Vulture in MMV.

[Grod_The_Giant]

I would just treat it as two separate encounters (assuming one minion).

If you have more, that's a bit more tricky.
More specificity, reinforcements. Not exactly two encounters. There's less chance to swarm the PCs, but also less chance to hit them with multitarget effects and a longer fight. I'd call it moot and stick both of them in the same characteristic.