Icathian Surprise When you die, you turn into a Kamikaze This is one of the most satisfying passives, it allows you to kill someone when they run happily with low HP when you die, but, most of the time if will make the enemy burn Flash or Heal. However, this passive also brings a negative consequence is that you need to die to use this, and it makes a little ineffective.
Caustic Spittle Debuffing skillshot and passive attack speed.This ability is very useful against tanks, this skillshots lowers Armor and Magic resist in a pecentage, and also provides passive attack speed, this attack speed is very useful at early game combined with your Bio-Arcane Barrage
Bio-Arcane Barrage Doubles your attack speed and Increases your attack range. This is your morning dose of steroids, you turn into a machine gun, this ability is essential for mid-game and late-game fights,This increases monstrounsly your DPS and allows you to take down turrets very easily and fast.
Void Ooze.Leaves a trail to slow enemies. This abilities effectivness will depend on your gameplay and deployment of the same. When you launches this ability is leaves a trail that applies magic damage to enemies and leaves a trail to slow them. Don't just use it if you are in panic, reserve it for the right moment.
Living Artillery Huge ranged splash damage. As it name says this is literally an Artillery, and the range of this ability is very useful for chasing the enemy. This is not a spammeable ability, the constant use of this ability will get you out of mana, also the damage is very reduced but its passives applies very high damage to champions under 50% and 25% HP, so this is basicly just used when the enemy is low Health

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We want to max out Caustic Spittle because it provides passive Attack speed Percent and also a high debuff. Then Bio-Arcane Barrage, this will be our main damage dealer and one of the most important late game abilities. We level up Void Ooze last as the first level slow is decent and it makes Oppressor work well, and you don't have to upgrade it. Then last but not least, you will max Living Artillery any time you can, this wont be very useful in the early game, it will be more a mana waste than a damage provider, but its passive its a really nice finisher if the enemy is really far.