CerealNommer

As I've pointed out in other comments there's a specific rule for damage past 0 HP. It's called “Damage at 0 Hit Points.” under the Death Saving Throws section, and it doesn't say anything about it getting cancelled out.

I would recommend somewhere non-vital, for instance, left buttcheek.

Yes, thank you. 👍

You can't "hit someone else"* because of the stipulations in contingency. RAI I'm sure they don't want you sucking the life from your left buttcheek to close up the mortal wound in your gut, but based on what they wrote, I haven't found a solid contradiction to this plan in the rules-as-written.

Also you can deal damage to something that is fully damaged. There's even a specific rule for "Damage at 0 Hit Points." under the Death Saving Throws section.

*You might actually be able to hit them but the spell at least can't effect them, so no damage, therefore no healing.

I mean, there's always a way to houserule something to fix the broken rules. Unless you're DMing an AL game.

The touch of your shadow-wreathed hand can siphon life force from others to heal your wounds.

Doesn't say "exclusively from others". Without casting this spell you couldn't do that normally.

It also says,

On a hit, the target takes 3d6 necrotic damage, and you regain hit points equal to half the amount of necrotic damage dealt.

No qualifiers about the target having to be a creature other than you. It just has to be a creature within your reach.

Also as regards:

As a DM there’s a few ways I’d consider to stop this being entirely game-breaking:

Do you also ban death ward and healing word? What about wizards who dip a level into virtually any other spellcaster or take a feat for a healing spell who can do this kind of thing without even having to make an attack roll or take necrotic damage?

Vampiric touch counts how much damage you deal, not how much hp the target loses.

It's definitely a flavor fail, but unless you houserule in a cap to damage dealt at 0 hp, (and houserules for how to handle massive damage and the rules for being dealt damage at 0 hp that would be affected,) there's not a limit to how much hp you gain based on the hp of the target.

You're still dealing damage, e.g. it could kill you from the massive damage rule if you dealt enough. Vampiric touch counts how much damage you deal, not how much hp the target loses.

Catastrophic dragons and Lung dragons really get shafted in these discussions.

My picks for each category

• Chromatic:
  • Green
    • Runner up: Black
• Metallic:
  • Silver
    • Runner up: Mercury
• Gem:
  • Beljuril
    • Runner up: Obsidian
• Catastrophic:
  • Blizzard
    • Runner up: Volcanic
• Lung:
  • Tun mi lung
    • Runner up: Chiang lung
• Planar:
  • Adamantine
    • Runner up: Shadow
• Top Honorable Mentions:
  1. Draa'zekyl
  2. Mist
  3. Prismatic
  4. Dragon Turtle
  5. Dracolich

I personally just rules lawyer back twice as hard.

• Well, technically revivify requires you to target a creature not the corpse of a creature…
• Actually detect magic doesn’t exclude itself, and the passive detection is pass/fail, so unless you’re immune to divination…
• Oh, the demi-lich got errata’d to have more hit dice, but they never “corrected” the default hp based on the special Undead Nature trait, so now it actually has %60 more hit points than the stat block would imply.
• Sure, freedom of movement lets you escape these non-magical restraints, but you have to spend 5 feet of movement to do it and your speed is currently 0.

A fair chunk That Guy’s tricks depend on a lenient or permissive DM/cherry-picking the most favorable rules & interpretations. If you don’t shadow-patch a bunch of fixes to make things work how people expect them to the game breaks down. If they want me strictly following the rules I’m happy to oblige, but I’ve never seen a powergaming munchkin who could withstand the amount of nerfing actually playing RAW causes.

Did I miss something or has no one even mentioned any One Roll Engine games?

I mean I love me some Fate, and PbtA is great and all. But where are the Better Angels? The Monsters and Other Childish Things? The Dirty Worlds?