Declare Fear of Missing Out as an attacker, triggering its ability, causing you to untap it and to get an additional combat phase after this one.
Deal combat damage to an opponent using Fear of Missing Out, triggering Impostor Syndrome, creating a token copy of Fear of Missing Out that isn't legendary.
When the Fear of Missing Out token enters, it triggers, causing you to rummage
During your next combat phase, declare Fear of Missing Out and the Fear of Missing Out token as attackers.
The Fear of Missing Out token triggers, causing you to untap the nontoken Fear of Missing Out and to get an additional combat phase after this one.
Repeat from step 2.
This infinite combat loop turns Fear of Missing Out into a relentless attacking machine, provided you've achieved delirium to fuel its extra combat trigger. Every time the original or its copies deal combat damage, Impostor Syndrome steps in to duplicate the threat, spawning non-legendary token copies that immediately trigger Fear of Missing Out's entry-level rummage. Meanwhile, Urabrask the Hidden ensures these fresh tokens jump straight into the fray with haste, bypassing summoning sickness so they can immediately attack, untap your board, and chain endless additional combat phases together.