Like the title says whats the risk of taking a warp claw SPECIFICLY from a dead possed marine

Im playing as a Warpsmith and i was thinking of taking this warp claw due to in the BC im in the partys lord died and we have his terminator armor plus 2 lighting Claws and a strange black gem.

I think it hosts a greater deamon of some kind

Anyway now would i get possed if i used a possed marines warp claw? Or would i just be unscaved

Hmm, no offence but I'm really struggling to want to give you an answer for this and would almost be tempted to tell you a flat no. It sounds to me like you're asking for information to help you metagame which I really despise of, asking for us to give you information out of character which will affect your in character decision. This is an RPG sin in my book...I would say play how your character would play and if you think he might be too focused on the wonder of the item and take it without heed to risk then he should do so since it is in their nature and to worry about the consequences afterwards. However, I can't force my personal opinions on the matter on others. I unfortunately do not have an army of commissars and inquisitors at my disposal. (yet)

I will however give advice anyway, perhaps I've misinterpret your intentions and plans and this is not what you are trying to do, not trying to get out of character information to change an in character decision. If that is the case of course then you have my apologies.

You need to decide what you want to do and need to research. First off, that's not enough information for us to work out if it's possessed. Black gems could be for anything and it could be decorative as well as functional, it depends on what the item is and how your GM has used it. For example if the lord had Slaanesh alignment it could have been a vanity item designed to make the claws look more resplendent but all the while with no actual other factor. You cannot reasonably state that it is or is not possessed purely because it has something shiny. Recommend that if you really do want to go down that road then you may want to have the item inspected, get someone skilled in Psyniscience to check for any warp trait on the item or use some of the divination powers to discern its true nature or purpose (assuming that the GM would allow that). The results of this will give you a better understanding as to its true nature. However for all you know the claws may not be the possessed item and it may actually be the terminator armour...if you are that paranoid you may wish to seek out archives of information to look up whether this item does exist as a named item, does it have history or a purpose? Would your skills with a book be able to help you discern its potential threat?

If you do go with it and equip a possessed item then you will have to battle for control against a daemonic item which is generally an opposed Willpower test with yours versus the daemon within the item, the value and relative chance of success with this will vary as it depends what the GM put in it. If you're lucky it'll be a minor warp entity and you should have a good chance of success. If you win then you bring it under control indefinitely so long as you maintain consciousness while wielding the item or your willpower does not drop to below that of the daemon within (or any lower than if already less) as in such cases the daemon may try again to assert control. If it is successful in the opposed test then it may make a possession action and use you as per the Possession Trait (BC Core Rulebook p142) and use you for its own ends until purged from you and will invariably have consequences such as (but not limited to and may be changed at the GMs discretion as thematically appropriate), permanent ability score damage, corruption and/or insanity points, mutations etc as well as any social consequences or winding up NPCs and own group.