Basically putting his ghost in his corpse makes him vulnerable. Probably best not to overthink it, but I'm running this in a Westmarches style campaign, and there's some lore that might explain it.
For what it's worth, his corporal form is basically trapped in a boss arena with the PCs.
This is all useful to know!
According the my Googling, the best way to gauge the CR of AD&D creatures is from their XP. So with that I've got:
- Shadows: CR1 vs. 1/2 in 5e; But given they can easily kill off a party member, I might even reduce the number of them to 1 (the encounter's basically an avoidable trap so the chance to get some strength drain on the Paladin might be enough).
- "Clutching Hands": CR1 (one CR1/4 per character); Basically a trap, so I'll run it as such instead of finding a stat block.
- Zombies: CR1/4 vs. 1/4; no changes needed here (plus the idea is that the party can just RE their way through the room instead of fighting them if they want anyway) =3
- Constrictor Snake: CR1/2 vs 1/4; I'll probably leave this at 3 to avoid a situation where every party member is constricted (the boss is a little higher CR than the original adventure, so it levels out).
- Giant Rats: CR1/8 vs. 1/8; I'll keep this the same (it's a dead end, and they run away if enough go down). There's an additional complication of normal rats swarming the room and getting in the way, which I'll translate as disadvantage.
- Spiders: CR1/2 vs. Giant Wolf Spider (1/4) or Giant Spider (1); the encounter has two, so I reckon if I have two wolf spiders, the action economy will make up the difference. Might throw in another if they seem to be doing too well at this point.
- Ghouls: CR1/2 vs. CR1: I'll keep two ghouls, but I'll have them spend the first round bowing/curtsying before they attack (their flavour is that they used to be servants), to give the party a chance to deal with them before they become too much of a threat.