Black Axe of the Monster (5e Equipment)
From D&D Wiki
Weapon (greataxe), legendary (requires a strength of at least 18)
NOTE: This is a more powerful weapon than usual. The idea is to improve the CR of an encounter with this weapon. If the DM allows, this weapon can also be used by a player.
Black Axe of the Monster is a monstrous weapon for monstrous creatures or players. As the name suggests, the blade of this axe is pure black and huge. The sharp side is very jagged. The handle is slightly curved, made out of an ancient magical tree and is dark brown with some steel elements.
While wielding the Black Axe of the Monster you have a +3 to attack and damage rolls. The weapon ignores overcoming resistances. It is useable as spellcasting focus.
Damage from Strength
To wield the weapon with both hands you must have at least a strenght of 18. If you want to wield it with one hand, you need a strength score of at least 20, regardless of any class features or feats. The slashing damage of this weapon is determined by your strength modifier. This means xd12 (x is your strength modifier minus 1) to a maximum of 6d12 slashing damage.
The magical power inside the axe releases its full potential. Once per turn on a critical hit, you can spend a bonus action to deal maximum damage (bonus damage granted by your class included). Furthermore, your critical hit range increases by one to a minimum of 19-20.
Power from Magic
If you succeed a saving throw to get only half damage from a spell attack, the weapon will put half of the taken damage into your next successful attack (rounded down). The extra damage type is similar to the spells damage type. (For example, if you take 45 fire damage, then your next successful hit deals additional 22 fire damage.)