Arcbound Overseer dies, it triggers, putting its +1/+1 counters on
Crystalline Crawler.
Activate
Crystalline Crawler's first ability three times by removing three +1/+1 counters from it, adding ({1} magic symbol) ({R} magic symbol) ({R} magic symbol) .
Activate
Aggravated Assault by paying ({3} magic symbol) ({R} magic symbol) ({R} magic symbol) , causing you to untap all creatures you control and to get an additional combat and main phase after this one.
Declare
Alesha as an attacker, triggering its ability, targeting
Arcbound Overseer.
Activate
Crystalline Crawler's first ability twice by removing two +1/+1 counters from it, adding ({WB} magic symbol) ({WB} magic symbol) .
Resolve the
Alesha trigger, causing you to pay ({WB} magic symbol) ({WB} magic symbol) to return
Arcbound Overseer from your graveyard to the battlefield tapped and attacking with six +1/+1 counters on it due to its modular ability.
Repeat.
Results
Infinite LTB.
Infinite ETB.
Infinite colored mana.
Infinite sacrifice triggers.
Infinite death triggers.
Infinite colorless mana.
Infinite combat phases.
Infinite untap of creatures you control.
Infinite +1/+1 counters on artifact creatures you control.