Abilities of Karthus
Spell skill priority is an important winning factor for Karthus. Understanding the abilities of Karthus in League of Legends is another. Below you can find all the relevant information on Karthus abilities. In order for item builds you should check the build for Karthus.
Passive: Death Defied
Upon dying, Karthus enters a spirit form that allows him to continue casting spells.
Lay Waste
Karthus unleashes a delayed blast at a location, dealing damage to nearby enemies. Deals increased damage to isolated enemies.
Karthus creates a blast of magic, dealing qdamage magic damage. If the blast hits only one enemy, it instead deals qsingletargetdamage magic damage.
Wall of Pain
Karthus creates a passable screen of leeching energy. Any enemy units that walk through the screen have their Move Speed and Magic Resist reduced for a period.
Karthus creates a wall that lasts for e4 seconds. Enemies that pass through lose e1% Magic Resist for e5 seconds and are Slowed by e3% decaying over the duration.
Defile
Karthus passively steals energy from his victims, gaining Mana on each kill. Alternatively, Karthus can surround himself in the souls of his prey, dealing damage to nearby enemies, but quickly draining his own Mana.
Passive: When Karthus kills a unit, he restores e2 Mana.totaldps magic damage per second to nearby enemies.
Requiem
After channeling for 3 seconds, Karthus deals damage to all enemy champions.
Karthus channels for 3 seconds, then deals totaldamage magic damage to enemy champions, regardless of distance.
FAQ on Karthus
What do you max first on Karthus?
Most Karthus pros max out 
Lay Waste first.