To get Metal Trident in King Legacy, you must defeat the boss Dough Master (Level 3275) at the Mirror Dimension of Loaf Island in the Second Sea. While the quest to defeat Dough Master has a level requirement of 3275, there is no requirement to fight the boss and obtain the drop. Metal Trident is an Epic Sword, so its drop rate should be around 1%.
Metal Trident is one of the tier-A swords in King Legacy, as it is mainly due to the heavy damage dealt by its two skills. Its M1 attack speed is slow, and it deals 4,501 damage with Armament Haki V1. Metal Trident is definitely not recommended in PvP as both skills deal a single blow, but it's quite good in PvE as it deals heavy damage, although the Z move is a bit suspect.
Its Z key move, “Sonic Leap” is unlocked at 1500 Sword Stat Points, and it deals 7,900 damage with Armament Haki in a single blow. This move requires you to place your cursor on the enemy otherwise it's very easy to miss or just lunge through the enemy. It's definitely not very consistent in its use.
Its X key move, “Meteor Fall” is unlocked at 3000 Sword Stat Points, and it deals one hit for 11,251 damage with Armament Haki. It's a very easy skill to use as it you will jump into the air and then deal AoE damage around you.
Note that you must have Armament Haki enabled for both Skills to work on Logia users.