How can I double damage, other than path of the grave?

Question

We're playing D&D 5e. I can't multiclass Grave cleric for roleplay reasons, so is there anything (feats, potions, other class features, spells, items, etc.) that has the same or similar effect as Path to the Grave, specifically the ability to double damage?

Channel Divinity: Path to the Grave

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.

Answer 1

The spell Contagion offers an option

Flesh Rot. The creature’s flesh decays. The creature has disadvantage on Charisma checks and vulnerability to all damage.

Unfortunately this takes 3 failed saves to apply, and requires a 5th level spell instead of a 2nd level class feature.

The Staff of the Forgotten One can also be used to:

Invoke Curse. The Staff of the Forgotten One has 7 charges and regains 1d4 + 3 expended charges daily at dawn. While holding the staff, you can use an action to expend 1 charge and target one creature you can see within 60 feet of you. The target must succeed on a Constitution saving throw (using your spell save DC) or be cursed. While cursed in this way, the target can’t regain hit points and has vulnerability to necrotic damage. A greater restoration, remove curse, or similar spell ends the curse on the target.

This is an artifact-type magic item.

I would say there is not a source of applying vulnerability that is as readily available as the Grave Domain Channel Divinity option.