Hey hey! So I've been pouring over the core rulebook and the Tome of Decay (hereafter called ToD) seeking answers but I can't find anything to guide me except my common sense, but that risks making this whole thing boring - I will elaborate:

Early in my groups compact our Chaos Space Marine Sorcerer decided to Push a power and rolled up Perils of the Warp followed by Grand Possession. A roll later and it turns out he's become host to a Lord of Change.

Now, here's what I don't get. The core rulebook says his strength and toughness now increases by 10 each, and he gets D10+3 extra wounds. Also the Daemons psy rating and powers should apply to him now, etc. That makes for a pretty powerful character! This is double when combined with the rules for possession in the ToD.

We agreed to have him keep playing the character, with all the conditions and rules found in the Tome of Decay - he was quite excited to have to battle his wills against the LoC for control of the body, so I am glad he takes to his new "friend" with such joy. I expect that will not last too long, but we'll see.

So the main questiosn I have here is:

Do the possession rules stack? If this is so, his character will become a super-powered 30 wound beast with strength 70 (power armour added on) and toughness 50, all the rules for using the Daemons Psy Rating, etc, as well as the rules in ToD.

Or does the ToD rules overwrite the core rulebook ones?

I find nothing on that in the book. I'd be happy to go with either option, but my worry is that this super-character will take the spotlight from the other players. This guy is headstrong and quite reckless with his characters, and this beastly thing is something he's really excited about. I do not want to bore any of my other players though. And I doubt playing such a strong character will be fun in the long run. Hell, it outshines a Daemon Prince player character. Not that big of a stretch with a Greater Daemon in the equation, but still.

I'm thinking of running with only the ToD rules, but if that is breaking the rules I do not want to do it. I will of course confer with my player group too, but I'd like some input to take with me to the discussion.

Advice would be appreciated. Thank you!

You either use core RB possession mechanics representing daemon "taking control utterly" (which also makes PC non-playable until daemon is exorcised or the host is dead) - which happends if you failed to resist possession attack caused by perils, wielding daemon weapon, etc, or Tome of Decay mechanics representing the creation of Possessed, a human who has daemon bound to his body and who is fighting a losing battle for control over himself (but still keeping the character playable most of the time) - which usually happends if the host undertook Ritual of Possession described in ToD.

Aha, that clarifies things. I think that is a good interpretation and a strong argument. Thank you.

For this case I think I will run with the ToD rules and let him keep playing the character. The LoC with it's high willpower - making it likely to win most of the battles of wills - and the fact that he'll be earning corruption very quickly from being host to it, should be punishment enough. He might be happy right now, but we'll see how long that lasts...