While this is a very good story, I have some unanswered questions.:
If Grune was alive in Jaga’s lifetime, why didn’t the other Thundercats know about him? This is very jarring when you compare Tygra’s comment in “Mechanical Plague” about learning from history.
Why was the Jaga in Cheetara’s flashback the same as the one we’ve seen in “Exodus” if this took place in Thundera’s past?
What happened to the Mauraders after Grune was banished?
I just watched it. My biggest questions are…
Why didn’t the Thundranium affect Grune? Since Thundrillium gave him strength, then thundranium should have weakened him. Yet he dug his hands into the pure thundranium and put it on his spiked club that he fought with.
If he was terrorizing Third Earth centuries ago and that was after he terrorized Thundera, then how does this fit in with the time frame? How long does a Thunderian naturally live? Or did Jaga some how find an answer to unnaturally long life?
Where was Mumm-Ra in this? Can’t see him contending with another evil force like that on his planet.
BTW, it was extremely wreckless and displayed horrible judgment of the ThunderCats to simply banish Grune like that, to allow him to then terrorize poor creatures on another planet with no one to stop him.
Well they’re bound by the 1980’s Cartoon Character Clause - They’re not allowed to actually kill anybody. 
But they could have trapped him in some kind of prison, like were Jaga was captured once (Astral Prison) 
They wouldn’t have to execute him. They could have put him in Thundranium shackles and isolated him in a prison cell for the rest of his existence.
What they did was very much like in Superman where the people on Crypton sent the three criminals into banishment until their prison was shattered and they found their way to earth to cause havoc. In fact, the device that Grune was banished in even kinda looked like that.
Thundrainium doesn’t work on Grune because Grune is not alive. He is a ghost like Jaga. That also goes for the “can’t kill him” thing.
I think I remember a later episode (not for sure) where he actually Came back to life and he couldn’t use his Thundrainum-laced club it DID affect him.
And from what I understand Jaga was centuries old.
Well if that’s the case, it should not have affected Jaga in this episode because when Grune battled Jaga, they were both ghosts, but Grune was unaffected by the Thundranium while Jaga was weakened by it, thus causing Lion-O to give up his strength to Jaga. They were both ghosts but only Jaga was negatively affected by the Thundranium.
Hmm.. You’re right. I’m not sure. Maybe it has something to do with the type of Thunderian he is. He’s one of the ones with more cat-like facial features.
Or maybe it could be because he is no longer a Thundercat and maybe that weakness goes away after his loyalties changed.
shrugs
Just speculation of course.
Honestly there probably isn’t a reason. The people who made the show have admitted inconsisties because they either didn’t pay attention or did things they thought seemed cool at the time.
I wonder if it was ever stated in the series if normal Thunderians are at all affected by either Thundrillium or Thundranium. Because they specifically say that Thundrillium is the source of the “ThunderCats” power. Might be a magic thing. So since Grune was no longer a ThunderCat, maybe it didn’t affect him? Seems a bit strange to me but it’s an aspect I don’t think they really ever explored as to why the two elements affect the ThunderCats the way it does.
I’m pretty sure Thundrillium is only ever described as a fuel (for the Lair and Tower, for their vehicles, etc.). Grune is the only example I can think of of a Thunderian drawing strength from Thundrillium. But then, in the same episode, we do see Lion-O somehow give up his strength to Jaga, and it’s represented by golden ripples of light, the same color as Thundrillium. Also, their eyes do glow that color when the Eye of Thundera calls for them. So maybe there is something to that.
As for Thundranium and Grune’s resistance to it, the allegiance idea is kind of cool, but it’s a little unrealistic. If Thundranium’s effect is biological, then personal allegiance shouldn’t matter. Personally, I like Cheezey’s idea from her fic “Path into the Darkness”, in which Grune receives a black market vaccine shortly before he dies. Or, at least, I assume that’s where she was going with it—the fic is still unfinished.
I always thought Grune’s resistance to Thundranium was due to his dabbling in dark magic. That might have been part of the reason he was banished rather than executed.
He dabbed into dark magic? Musta missed that part. 
I don’t think you missed anything. The only reference to magic in that episode is the magic seal that kept Grune from leaving his tomb (“broken by those fools!”). But that would be magic wielded not by him, but by whoever defeated and trapped him there. Right?
My earlier post isn’t official cannon; just an opinion.
I think your opinion, Lady Ocelli, is good. I can defintely see where Grune could have pursued learning magic and one more powerful than him chose to imprison him for whatever reason.
If you recall in “Return to Thundera” Mumm-Ra summoned Grune to face Lion-O and Cheetara at the Churning Rocks. Grune’s first words weren’t “Who are you?”, they were “Mumm-Ra…”
I don’t remember Grune being in “Return to Thundera.” Lion-O was accidentally transported there by the time capsule and during his absence, the Mutants with Mumm-Ra’s help unleashed the warbot on Cats Lair.
I believe LioConvoy meant “Return to Thundera!” the five-parter, as opposed to “Return to Thundera”, the standalone ep from Season 1. Grune is summoned by Mumm-Ra in Part 2.
Yup, that is indeed where I was going with that. I still intend to finish that story someday. Also, I’m flattered that you remember my story. It’s been years since I posted it. 
Also, regarding the other ThunderCats having should have known Grune from their time on Thundera, perhaps he was there in Jaga’s younger days, while the other TC’s were kids (or not even born yet), and they just didn’t show Jaga younger in the flashbacks. That’s how I was reconciling it when watching it today, lol.
Part of the reason I wrote “Unexpected” was because Grune’s banishment left too many unanswered questions in my imagination.