What happens when the unit on the board that I Showed Off with Primordial Roar is returned to my hand with Star-Crossed? Do I still deal damage? What about if I play Lunar Boon and discard the unit from hand?

You still deal damage in both scenarios.

Any referenced characteristics of shown off objects are locked in when you show them off. Regardless of what subsequently happens to them, their information will be able to be checked by the card that showed them off. This also means that giving a unit +Might or -Might after showing that unit off will not affect the damage dealt by Primordial Roar; it’s too late by that point.

Riot's question and answer, word for word, from theCore Rules: Radiance Patch Notes (Effective October 16, 2026)·Oct 9, 2026·Core Rules 832.1.d.1 (as numbered in October 2026)