Companion that deals necrotic damage?

[Arkhios]

Is there an official(-ish) way to obtain a permanent companion that would deal necrotic damage?

I suppose a refluff of an official option would qualify as "-ish", although not "B/P/S to Necrotic", because they're not within same categories (physical vs energy).

[CheddarChampion]

Get a sidekick (UA) mage that can cast necrotic spells?
Befriend a mage that can cast the chill touch cantrip?

What do you mean by companion?
Class feature? DM-granted boon?

[JackPhoenix]

Hexblade level 6 ability gives you a specter once you kill someone. Not permanent, but it'll last until you take long rest, which, with some coffeelocking can be never. Necromancer wizard can take control of an undead at level 14, and, if the undead in question has Int <12, keep it controlled indefinitely. Shadows, spectres, ghosts and vampiric mist does necrotic damage, other undead either have necrotic option or combine necrotic and physical damage.... vampire spawn, mummies, wights, bodaks.

[Arkhios]

Urgh, forgot to respond earlier.

My fiancée wants to make a character with a frog companion that could deal necrotic damage somehow (most preferably with their tongue, but not entirely mandatory).

DM gave a few suggestions:

Beast Master, taking a Giant Frog as the beast companion (swapping acid damage from being swallowed to necrotic damage)

Chain Pact Warlock, taking Quasit as familiar (swapping the additional poison damage with necrotic damage; quasit can apparently shapechange into a Toad, and use quasit's claw attack as a bite instead)

She decided to go with the Beast Master (and honestly, so would I). It would've been fun if the frog could have some other necrotic melee attack, but I guess if she's happy with the swallow damage change, no need to make things more complicated than that.