catastor vs grand mole

well i wanted to ask about this situation:

WHICH EFF WILL RESOLVE FIRST???

this is the situation:
i have catastor on field and my opponent grand mole
im on turn and i atk
both effs activate at the start of damage step but i think there is a general rule for this

(i always thought catastor will win cuz i have priority)

catastor and grand mole both have an effect that triggers simultaneously. at the start of the Damage Step, however catastor has a Mandatory effect meanwhile mole has a Optional Trigger effect. So no matter who controls them, Catastor will be chain link 1, Grand Mole will be chain link 2, meaning Grand Mole will return itself to hand and Catastor to
the Extra Deck before Catastor can resolve.

confirming Renji. chain will ALWAYS be:

link 1: Catastor
link 2: Grand Mole
chains resolve backwards.
Mole resolves sending itself to hand and Catastor to Extra Deck.