Grandmaster of the Great Fang Group, Fat Fang
My opponent tried to enter the battle phase when I had "Grandmaster of the Great Fang Group, Fat Fang" on my opponent's field and a unit in my opponent's Graveyard. I have a removal that can be cast (Mercurial Light March), and there should be a timing to hit this removal before entering the battle phase, but without that, I will arbitrarily say "Chief of the Great Fang Group, Fat Fang". induced.
Probably, the conditions such as "Originally Fat Fang was in play", "Originally the machine was not in the graveyard", "The machine was sent to the graveyard by the routing ability", "There is no other action that can be taken by the opponent", etc. I'm guessing that it has progressed to the trigger of "Chief of the Great Fang Group, Fat Fang".