Karthus- Why You NEED To Get Dark Seal/Mejai's On Him
Dark Seal
+15 Ability Power
+25% Healing From Potions
+100 Mana
UNIQUE Passive- Dread: Grants +3 Ability Power per Glory.
UNIQUE Passive- Do or Die: Grants 2 Glory for a champion kill or 1 Glory for an assist, up to 10 Glory total. Lose 4 Glory on death.
Mejai's Soulstealer
+20 Ability Power
+200 Mana
UNIQUE Passive- Dread: Grants +5 Ability Power per Glory. Grants 10% Movement Speed if you have at least 15 Glory.
UNIQUE Passive- Do or Die: Grants 4 Glory for a champion kill or 2 Glory for an assist, up to 25 Glory total. Lose 10 stacks of Glory upon dying.
Karthus is currently a strange yet powerful champion in the game known for one thing... that late game. Karthus is an absolute beast late game due to his insane damage from his Q and his Ultimate that forces the enemy team to either build around him or lose a huge chunk of their health if they don't. However, even early and mid-game Karthus has his uses, particularly in his ultimate. Karthus, despite being an immobile Mage, can be a very safe pick if played correctly. His Q essentially being a basic attack in lane allows him to easily last hit from a distance and harass if he has Blue Buff especially. However, a common mistake I see on Karthus players is that they build Doran's Ring instead of Dark Seal. There is only one, maybe two situations where I would say Doran's Ring is a better choice on Karthus than Dark Seal with the main being if majority of the enemy team can innately negate his ultimate (ex. Nocturne, Sivir, Fiora, etc.). Otherwise, Dark Seal is simply incredible early game. Why? His Ultimate can give him so many stacks for simply pressing R when ANYONE on the map is near death, even if Karthus isn't the one to kill them.
Now, I can already here people say "But Inept, Mejai's and Dark Seal lose Glory on death and part of Karthus' play style IS dying. Why would I want an item that makes me lose power when dying?" Well, then you do not realize WHY Karthus dies in a team fight. A Karthus does not simply die to die in a team fight, he dies to get kills or assists. Anyone can die, but Karthus is one of the few champions in the game that is AP and can take back his stacks of Glory that he lost upon dying. Died? No matter, get 2 kills and an assist in a team fight and you made up for it. Plus, Dark Seal/Mejai's provides a TON of AP for the gold cost if you play Karthus properly and allows you to have a ton more AP when building Karthus, especially since he usually builds AP items that are more on the defensive side.
So, essentially what I have come to tell you is that Karthus makes good use of the Dark Seal and, in all honesty, I think it is about time the community experimented more with this item, especially now that the LCS has begun to switch back to normal lanes. Normal lanes allow Lane Bullies and Lane Bullies can stack these items much more reliably than other champions. While Doran's Ring is still something I would take on mana hungry early game champions, I think it is definitely worth looking into the Dark Seal simply due to the early lead it can provide, especially if you know your champion will win early trades against enemies OR has an easy way to get a constant stream of assists throughout the game.
. Assist ult to stack up, then use your gold to get that quick deathcap for earlier high damage ults and such. His damage is way too low these days without some quick dmg boosting; at least from my experience.