So I went to Hepaetus with Grumpos and one other, wandered around, did a lot of stuff quest-wise and such but did NOT fight the Levant Boss. Then went back to Sender Station Station to regroup and do a little leveling with other members, and tie up a few loose ends, while I pondered the party to take back for the Levant Boss (and get that huge XP from the battle).

So now I've decided to take Grumpos for sure but he WON't JOIN! I only get a dialog about how important it is to talk with the Grand Mysterium and what a hotty Stiletto is.

Anyone else have this conundrum? Or anything similar? I wasn't able to find any historical discussion of this and since GOG has the most active forum (that I surmise) thought I'd post here.

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I think I have a way figured but it requires the debug console. I generally only use a console for dire cases (such as to overcome bugs). Is the fact Grumpos won't join the party at this juncture a bug? I don't know but I suspect so. Will forcing a party with him in it crash/spoil the game? I sure hope not.
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<edi2t>
So I did the evil and forced Grumpos into the party using console commands. So far so good.
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Well he IS supposed to be on Hephaestus since he wants to check out the library at the tower. So I can't imagine it would crash the game.

I suspect no one else noticed this because no one else ever thought to go back to Sender Station Station before the chat with the Grand Mysterium. After that mission you get plonked back on SSS, and at that point Grumpos won't join you for any other missions for a while.

Grumpos has to be there for the library part, but that's over before you go into the Levant. He doesn't HAVE to go in there; yammer is not required and in the end only Boots gets to chat with the Grand Mysterium anyway.

There is probably a variable in the game that updates once the library part is over, that "grounds" Grumpos when he gets back to Sender Station, since he's not required anymore. This "update" should probably only happen after you chat with the Grand Mysterium, since it makes sense that Grumpos should stick around for that; but apparently it doesn't.

You're right, no one else noticed this because no one else ever thought to go back to Sender Station before the Levant, probably because we all assumed Grumpos was still required or that we couldn't leave Hephaestus before we talked to the Grand Mysterium.