Looking For A Baddie

[batsofchaos]

For a campaign I'm starting, the BBEG is going to be an incorporeal undead. The key feature I'm looking for them to have is the ability to possess living creatures for what is potentially an indefinite amount of time. I was originally thinking Ghost, but the Malevolance ability just functions as a Magic Jar spell using the Ghost's HD as caster level (minimum 10) for the duration, so having the thing at 20th level means that they can only possess a target creature for a maximum of 20 hours, which isn't cutting it for my needs. Would the best thing be to houserule a more permanent form of Malevolance (with a modified Save system so the target creature has a chance to resist as time goes on), and up the CR/LA for the template? If I did what would be the new CR/LA? Or is there another undead (preferably template) that has a power that fits my needs that I'm unaware of?

Thanks!