"Navi, this is a very important warlock artefact!"
"This is a really important lore piece!"
OK OK. All so Illidan consumed it and gained Gul'dan's powers, I was thinking that was the extent of it. And yes, Gul'dan was a very important orc warlock, perhaps the most powerful mortal warlock ever. I though that was why they were so keen to have it.
However, that wasn't just it! What made it so COOL was the green fel glow you get when you activate the on use ability of the trinket, which makes you glow green for 20 seconds.
So this is what you look like when you have Fire kitty and you activate the trinket:
Cool right? Heh, I think so! I did it during raid time just to be funny and people wondered how I went green.