Venerated Rotpriest does not proc Fynn, The Fangbearer's ability
When I played Venerated Rotpriest first turn, then Fynn, the Fangbearer second, I swung with Rotpriest and it did damage to the opponent. But, it only gave them one poison counter instead of three, one for Rotpriest normally and plus two for Fynn's passive ability.
1
vote

-
Rezzahan#77802 commented
Venerated Rotpriest does not have deathtouch, so Fynn doesn't matter here, he only cares about creatures with deathtouch dealing combat damage. Only 1 poison from the Rotpriest's toxic 1 is therefore correct.