All In is a weapon specific skill in Divinity: Original Sin 2.
In game description[]
Perform a heavy attack that deals X-Y physical damage.
Damage is based on your basic attack and receives a bonus from Strength or Finesse (depending on the type of weapons used).
Description[]
It's a special skill that can be used with two handed melee weapon. It appears automatically when this weapon is equiped in the main hand slot. This will not work with staves, because they have their own specific skill.