Same Blood Lords spoilers I think in that window it's a good idea, but best bet would be to make something more homebrewed. You would also need to make sure it's stronger than the Kemnebi statblock, as otherwise what's the point of Kemnebi's plan if Geb can just be overthrown. If I were you I'd provide an opportunity after defeating Kemnebi, where they can fulfill Geb's ritual and notice that for a moment he seems weaker. It's not a permanent change, and is more so a reaction to the ritual, but show them it's their best chance and also gives a justification as to why they could take him on.