Vaal Oversoul

Vaal Oversoul is the final boss of Act 2, located in Ancient Pyramid

One must use the Apex, or complete amulet from the quest Deal with the Bandits in the Dark Altar in order to summon it.

Defeating it opens the way to Act 3

Previously in the Beta as of 0.9.12, defeating it opened the way into the next difficulty.

Abilities

Lightning Beam

Fires a lightning beam with a 100% chance to cause Shock on hit. One cycle contains 3 shots.

Ice Shards

Shoots ice shards that spread ground ice on the floor, causing the players walking on it to become chilled.

Slam

A powerful slam to the ground, dealing massive damage in an area (50% Physical, 50% Fire) with increased critical chance.

Summoning

Summons monsters.

Hide

Hides in the ground for several seconds and then emerges in a different location. Triggered after the cycle of summoning monsters.

Quake

Causes rocks to fall from the ceiling in a random pattern, dealing high physical damage in a small area.

Strategy

The best counter to its powerful attacks is good mobility. Always pay attention to its movements and see which arm it is using. A good way to avoid sudden spike damage (beam & slam) is to back off and start moving before the cycle of its current ability has finished. This way you are in a safe position to counter its next choice of ability.

The longer, thinner fork arm shoots lightning beams and ice shards. Dodging the lightning beams is important, as it causes Shock on hit and can stack up to 3 times, causing you to take 40% increased damage per shock from all sources. Having maximum Lightning Resistance at this point is a good safety net, even though the shots are very easy to dodge with simple circling maneuvers. You should try to stay away from the ground ice as it slows down both your movespeed and attack/cast speed, weakening your position considerably.

The bigger ball arm is used for the quake and slam abilities. The slam is slow and relatively easy to dodge. Having very high Armour and fire resistance can help you survive a hit. However, trying to tank it is generally a bad idea as it can deal unsurvivable damage on higher difficulties. The falling rocks are relatively hard to dodge if they happen to fall near you due to the short time frame you're given to react. A small amount of dust and gravel will fall as a hint for the location where a bigger rock will fall shortly after.

The third arm is used to summon monsters. The ranged monsters it summons deal high physical damage and chaos damage. The melee monsters are slow and can inflict elemental status effects on hit. A good time to take care of them quickly is when the boss hasn't yet reappeared. While taking care of the monsters, it's a good idea to be ready for when the boss reappears as it can sometimes almost immediately start its next choice of ability. The summoning cycle can be repeated immediately if enough of the summoned monsters have died before the end of the cycle.

Causing the boss to get stunned, while extremely hard to execute considering its life pool, can interrupt its current ability and initiate the next one. The boss can generally repeat most of the abilities multiple times back to back.