I'm no Skarner main, but I feel like his ult is outdated and outclassed by dozens of abilities in the game. Abilities that aren't even ults.
I think his ult should be changed to something...more scorpion like.
R- Crystal Venom Sting
Deals 350/700/1050 true damage to minions/monsters, instantly. 50% CD if used on minions/monsters.
Inflicts enemy champion with Crystal Venom. After 5seconds, enemy champion will Crystalize for 5seconds, becoming immune to everything and unable to move(Like Zhonyas, but with a prettier animation). While Crystalized enemy champion takes 10/20/30% of maximum health as True Damage.
200 cast range.
How would it be used?
Primarily as a tank disabler. R the enemy tank, so your team can go in and make a play.
the flat unscalable true damage on his ult, allows him to do relevant damage, without having to build damage, which I think is important considering his kit barely has room to consider building damage.
Also he isn't that fantastic of a champ, a pseudo Smite through his R would bring some value to having him for Objectives, like with Chos feast.