Prerequisite: Ability to deal lightning damage

Tesalic Lightning
Whenever you deal lightning damage with a weapon attack, cantrip, or spell, it is automatically converted to Tesalic Lightning — a rare and volatile current that counts as both lightning and necrotic damage.

• Resistance Rule: If a target is resistant or immune to either lightning or necrotic damage, the Tesalic Lightning damage is halved or negated as appropriate.

• This applies to all lightning damage you deal; there is no way to turn the effect off.

Necrotic Surge (Always Active)
The Tesalic Current tears at your own vitality:

• Each time you deal Tesalic Lightning damage, you take 2 unpreventable damage.

• This self-damage ignores resistances and immunities.

• If you hit multiple creatures with a single attack or spell, you only take this self-damage once per attack/spell, not per target.

Corruption Over Time (Weakened at Level 1)
The energy of Tesalic Lightning lingers painfully after combat:

• At the end of your turn after using Tesalic Lightning, you must succeed on a DC 10 Constitution saving throw or take 3 additional unpreventable damage.

• This repeats each round for 1 minute after the last time you dealt Tesalic Lightning damage in that combat.

• These effects end early if you drop to 0 hit points, are knocked unconscious, or finish a short or long rest.

Scaling Plan
This feat evolves as you grow in power:

• Level 5: Necrotic Surge self-damage increases to your proficiency bonus. Weapon infusions gain +1d6 Tesalic Lightning damage.

• Level 7: Creatures that take Tesalic Lightning damage must succeed on a Constitution saving throw (DC = 8 + proficiency bonus + your spellcasting ability modifier) or be stunned until the end of their next turn.

• Level 8: Unlock Siphoning Shock — when you deal Tesalic Lightning damage, regain hit points equal to half the damage dealt (rounded down). This can be used a number of times equal to your proficiency bonus per long rest. Full corruption rules now apply.

• Level 15: You may choose to ignore resistance to either lightning or necrotic damage when dealing Tesalic Lightning damage. Only one resistance type can be ignored at a time.

Lore

The art of channeling Tesalic Lightning, otherwise called “the lightning’a life,"  is a cursed gift, known only to maddened sailors and sea-folk who dwell in the grip of lunacy. Feared by those who’ve glimpsed it, this power is whispered of rarely in ports and harbors, its origin traced to a flower found only in the blackest reaches beneath the waves, far below where even the sun dares shine.
Beneath the waves, the Impeaye’fleur forms an oxygen-rich membrane around itself, a shimmering bubble of breath. Yet this air is not like the wind above; it carries a heavy, drugged weight, leaving those who inhale it half-dreaming; drifting between wakefulness and the dark depths that claim the unwary.
You have bound both body and soul to the Impeaye’fleur, a rare bioluminescent bloom whose pulsing core generates a current unlike any other, one that marries lightning’s swiftness with necrotic decay. This unnatural current draws upon your very life-force, feeding the flower and granting it an undying existence. Some say a single stem can grow for thousands of years, its age marked by the slow inches of its stalk.