I have suggested many Skarner reworks/partial reverts, and Crystal Venom was a thing I used in them:
Passive: Crystal Venom
Every 6/5/4 attacks (levels 1/6/11) Skarner strikes with his tail, dealing X% of his targets max HP over 1.5 seconds. For the duration, a target inflicted with crystal venom is weakened, dealing Y% less damage.
Q: Crystal Slash (Pre-juggernaut) Base damage plus bonus ad scaling, with additional magic damage while charged. Charges last 5 seconds and all fall off at once, can hold up to 3 charges, charges also grant MS/AS per stack.
W: Crystalline Exoskeleton (NEW-ish) Skarner charges his body with Crystal Energy, granting him a shield, tenacity, and increased movement speed for up to 6 seconds as long as the shield holds. Skarner shield has a base value, scales with max hp and AP.
E: Fracture (newish) Skarner fires a beam of crystal energy dealing magic damage and slowing all targets hit, marking them for 5 seconds. If Skarner auto attacks a marked target, they are stunned for one second and restore mana to skarner, increased vs champions and large monsters.
R: Impale(Same-ish) Skarner imaples a target dealing damage/supressing/dragging etc. When the suppression ends (cleansed or duration ends) the target takes additional Magic Damage and is affected by Skarner's Crystal Venom.
Very simple, but effective rework.