This trident is a magic weapon. It has 3 charges. While you carry it, you can use an action and expend 1 charge to cast dominate beast (save DC 15) from it on a beast that has an innate swimming speed. The trident regains 1d3 expended charges daily at dawn.
RollQuest ships the D&D 2014 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.