Eelskin

2nd-level biomancy*

Casting Time: 1 action or 1 bonus action
Range: Touch
Components: V, S, M (some jellied eel)
Duration: Concentration, up to 1 minute
Class: Druid, Sorcerer, Tamer, Warlock, Wizard

You cause the skin of a willing creature you touch to produce a slimy, electrified mucous. The creature’s unarmed strikes and natural weapon attacks become magical for the duration of the spell. The first time the creature hits with an unarmed strike or an attack using a natural weapon on its turn, it deals an additional 1d8 lightning damage as the stored lightning discharges.

Slimy. For the duration, the creature has advantage on checks and saves made to avoid being grappled or restrained, and disadvantage on any attempts it makes to grapple another creature.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d8 for each two slot levels above 2nd.

*can be replaced with evocation
Source: Heliana's Guide to Monster Hunting