Tips for Karthus from a D5 Karthus main

I am not a Diamond Player, but a while ago I visited the League Subreddit and came across thread from a Mastery 7 Karthus main (Silver 4 I think?) and a D5 Karthus came and gave some tips on how to get better playing Karthus. I figured I'd try to summarize what he said and post it here for everyone else who'd like to play Karthus. (I'd do direct quotes, but for some reason the Quote block isn't working at points atm :/ )

Here's the Source if you want the unsummarized version

1. Dying as Karthus is fine as long as you have good positioning.

2. Don't build more than 1 defensive item on Karthus. You need the Damage

3. Void staff is important on Karthus

4. Get CDR on Karthus

5. Try not to push too far without Vision

6. Coordinate your ult with other lanes to help them transition better into mid/Late Game

7. Try not to far with Q early game if you can. Save the mana for Zoning/Harassing the opponent. Don't make it too much a priority though

8. Pulse E to kill Minions if you can't Q or auto in time (Quickly turn on and off the E)

9. It's not super important to get boots on Karthus, but get them if you need to help avoid damage. Sell late game for more damage

One thing he did say about starting build on Karthus:

I usually start Doran's ring, it allows you to get so much mana back from killing creeps with it's passive + your E passive. The extra health also helps vs a lot of champs that want to burst you down (which is a lot of the current meta mid-champs). I do sometimes go Corrupting potion as a starting item, if the enemy can outrange with poke (Xerath, Varus, Brand...), to sustain through all the harass in lane.

I don't like starting Dark Seal personally, but it is honestly fine as an opener. You don't really need the mana regen that you'd get from Doran's or using Corrupting Potion since you have E passive, as long as you can last-hit decently well with auto's. The biggest reason I don't like starting Dark Seal is because I don't think Mejai's is an ideal item on Karthus. Of course it's not bad, as you can stack up a lot off free AP off ulting for kills/assists. But, sometimes in the mid-late game 5v5 teamfights, the best thing you can do to win the teamfight can be to jump right in the middle of the whole thing and just suicide while cranking out crazy DPS - which is pretty counter intuitive for Mejai's. Also, you are pretty prone to getting picked and dying since you have no mobility outside of Summoner Spells.

Small point I wanted to bring up about biscuits: they are good if you need the small extra health/mana that buscuits give over regular potions - but, if the enemy mid laner is a champ that doesn't have a crazy amount of kill pressure in lane, I like to greed and take Assassin over biscuits in the mastery tree. Karthus certainly isn't an assassin, but a lot of the time you are ulting, you are not near any of your teammates, so you get the extra 2% damage buff that Assassin gives (which is a decent chunk of extra damage since your ult does so much).