Does anyone know which challenger drops the loot ?

I just found out my outrageously priced stash of Tiger Bones is getting bought. It's likely I haven't finished all of my challengers since the Battle Coloseum reset, so it's possible I can just stomp a few more out and don't have to worry about it. But if it was an event challenger then I will want to get a few more Exodin made before I let them get bought up.

El Tigrean drops it, I don't think he will be back tho

oh ha ha I guess not. Thank you!