Executioner has a random chance to be the dungeon boss in Dungeon of Island/Meadow/Wild. I believe you need to be on island level 5+ for the boss to spawn (random chance). The boss is quite challenging using variety of strong attacks.
If you're struggling with the fight here's my one shot build (Deals about 60-70k per hit)
2x 400+ ATK weapons
Combat skills:
Charge strike - lvl2
Battle hymn, dual wield, berserk, backwater formation, unstoppable, lai slash - all max lvl
Lean force - lvl1
Wild Might - max level
Before the boss room eat raw meat and apply all 3 buffs, come up to the boss and use Lai Slash, boom 70k damage. Warning you're extremely vulnerable with the buffs on.