NOTE: MY ENGLISH IS HORRIBLE , I FOUND BEST USE TRANSLATOR FOR BETTER COMMUNICATION , SORRY FOR ERRORS

PROS

.good clear wave
.his ult is one of the best (if not the best) in lol
.great cc
.its passive works very well with his ult
.big sustain with E passive
.many players They do not know how to play against karthus
.PASSIVE ALLOWS YOU TO USE ANY SKILL AND MAY NOT BE CANCELLED

CONS

.low mobility
.Squishy against assassins/burst
.use too much mana in early game
.if E passive is not used properly,will use much mana
.very late game

speel required for Karthus

exhaust is one of the best speels for Karthus in my opinion , reduces armor and thereby amplifies the damage of Q

It is a good speel if you want a good fight , also works great with seraph

It is good against mid laners with HARD cc .
an example: AHRI if she uses the taunt , she will use all your kit to kill you , using this speel you completely cancel the combo, this speel also cancels ignite

farm with Karthus is simple , use the Q in Minions , but use 1Q to 1 minion , so the damage to minions is increased.
The passive E recover mana by killing, then use the Q carefully
AVOID USING E to farm , this skill uses a lot of mana in early game .