You have to agree to them having a cut of any inheritance and then they set about proving you are heir to some far distant relative you've never heard of.
A friend has been contacted by two different companies regarding the same "relative". Presumably these companies know there is money to chase?
No one has ever heard of the relative and one sibling has done considerable family tree research without coming across them, but I suppose that's the point, they'll be on a distant branch (if connected at all).
Would you get involved or assume it's a scam? Obviously it's going to mean sharing a lot of personal data.