You gain a +1 bonus to attack and damage rolls made with this magic weapon. In addition, while you are attuned to this weapon, your hit point maximum increases by 1 for each level you have attained.
Curse. This axe is cursed, and becoming attuned to it extends the curse to you. As long as you remain cursed, you are unwilling to part with the axe, keeping it within reach at all times. You also have disadvantage on attack rolls with weapons other than this one, unless no foe is within 60 feet of you that you can see or hear.
Whenever a hostile creature damages you while the axe is in your possession, you must succeed on a DC 15 Wisdom saving throw or go berserk. While berserk, you must use your action each round to attack the creature nearest to you with the axe. If you can make extra attacks as part of the Attack action, you use those extra attacks, moving to attack the next nearest creature after you fell your current target. If you have multiple possible targets, you attack one at random. You are berserk until you start your turn with no creatures within 60 feet of you that you can see or hear.
Proficiency with a Greataxe allows you to add your proficiency bonus to the attack roll for any attack you make with it.
This weapon has the following mastery property. To use this property, you must have a feature that lets you use it.
Cleave. If you hit a creature with a melee attack roll using this weapon, you can make a melee attack roll with the weapon against a second creature within 5 feet of the first that is also within your reach. On a hit, the second creature takes the weapon’s damage, but don’t add your ability modifier to that damage unless that modifier is negative. You can make this extra attack only once per turn.
Notes: Bonus: Magic, Bonus: Hit Points per Level, Combat, Cursed, Heavy, Two-Handed, Cleave
What is the attunement requirement.
And do you go berserk once someone takes your axe
I would rule since the cursed creature is unwilling to part with it, there would be a contested strength check. But should it be stolen or forcibly taken they would need to roll the wisdom saving throw or go berserk. Either way unless a remove curse or similar is used on them I would say the affected character would do what they could to get it back.
Typically unless the notes say otherwise it takes at least a short rest familiarizing themselves with the item. In this example perhaps spending the short rest working through movements with the weapon learning the feel.