What ways can an enemy be made to take more damage?

[CheddarChampion]

There are many ways to make your attacks deal more damage to an enemy (Hex, Hunter's Mark), there are many ways to make a character deal more damage to any enemy (Divine Favor, Elemental Weapon), but what ways can you make an enemy take more damage from any attacks?

I know of two ways already: Hold Person/Monster (extra damage from crits) and Channel Divinity: Path to the Grave (vulnerability for one attack). Are there any more effects like these?

[Khosan]

Contagion - Flesh rot is a very slow option. Target has to fail 3 Con saves, but if they do they're vulnerable to all damage.

[LostBenefit]

The Grave Cleric's Channel Divinity: Path to the Grave in Xanathar's Guide to Everything,


Starting at 2nd level, you can use your Channel Divinity to mark another creature’s life force for termination.

As an action, you choose one creature you can see within 30 feet of you, cursing it until the end of your next turn. The next time you or an ally of yours hits the cursed creature with an attack, the creature has vulnerability to all of that attack's damage, and then the curse ends.

[Jack Bitters]

Not quite an amplifying effect, but if you stun someone, they automatically fail Strength and Dexterity saving throws. Stun -> Disintegrate might be worth looking at.

Elemental Bane is a 4th level spell in Xanathar's that removes resistance:

Choose one creature you can see within range, and choose one of the following damage types: acid, cold, fire, lightning, or thunder. The target must succeed on a Constitution saving throw or be affected by the spell for its duration. The first time each turn the affected target takes damage of the chosen type, the target takes an extra 2d6 damage of that type. Moreover, the target loses any resistance to that damage type until the spell ends.