if I use emperor dais to return a unit after conquer and play a sand solider, will rengar pridestalker trigger?

Yes. Playing the Sand Soldier token from Emperor's Dais counts as "playing a unit," so Pridestalker triggers and you give a unit +1 Might this turn.

Tokens aren't Main Deck cards, but they are units that are "played" when created by an ability that uses the word "play" — which is exactly what Dais does (Rule 350.2; FAQ #9982, #10901). This holds even if you returned your only unit as the cost: Dais pre-selects "here" as the token's location, so it still plays to the battlefield (FAQ #3615). When the trigger resolves, you can target any unit — not just the Sand Soldier — for the +1 (FAQ #9769).