Since my polymorph meme has only garnered three downvotes so far I thought I'd offer a bit more controversial take, and see if I can manage to stir the pot a bit with this one.
So this sent me down an absolute rabbit hole. As a DM there's a few ways I'd consider to stop this being entirely game-breaking:
You could argue that the only thing strength before death shows is that you can activate strength before death between hitting 0 and getting knocked out. A wizard is no samurai. Therefore concentration spells are not allowed.
You could argue that life steal requires life to steal, and as such you can't life steal yourself.
You could enforce the requirement of the figurine required for vampiric touch, then engineer a scenario to remove it at a critical moment and see if they realise.
Personally I would instead depart from RAW and point out a version of option 2, but a lenient one. Something like "you can do this but you are sapping your very essence to do it. Every time you do it, you permanently lose 10% of your HP" or "every time you do this you increase the number of death saving throws you must succeed before you die". Or my personal favourite: "every time you do this you perturb the very laws of nature. Nature is rather fond of its laws and so decides to perturb you right back. Roll on this table to see what happens." and make the table include the above alongside a few other things and maybe a roll on the wild magic table.
In the end I enjoy ingenuity but the role of DM gives you a lot of latitude to... handle... those who believe they found a loophole.
Nature is rather fond of its laws and so decides to perturb you right back.
"Your character has started to understand and unlock the secrets of undeath. Do this again and you may need to find a phylactery quick, because each time moves you one step closer to becoming a lich"
The touch of your shadow-wreathed hand can siphon life force from others to heal your wounds. Make a melee spell attack against a creature within your reach. On a hit, the target takes 3d6 necrotic damage, and you regain hit points equal to half the amount of necrotic damage dealt. Until the spell ends, you can make the attack again on each of your turns as an action.
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?
They're not ignoring it, they're focusing on the part that says they can siphon from others. It doesn't explicitly say they can only siphon from others. RAI obviously wouldn't allow it because hurting yourself to save yourself is ridiculous, they're trying to say that RAW isn't specific about this particular use case and therefore it's OK. Which is... A stretch at best.
It doesn’t explicitly say they can only siphon from others.
And it doesn't explicitly say they cannot fly yet no sane person would infer that this spell makes you capable of flying. The spell does only what it says it does, otherwise you are still bound by all the base rules and limitations of the game, like not being able to willy-nilly siphon life force out of things without a feature that explicitly says you can.
Except it does say “from others”. Highlighting “can” just shows that this spell gives you the ability to do so. Another way of writing the text would be “The touch of your shadow wreathed hand gives you the ability to siphon life force from others”
I'd possibly go for it as well, Except there would be a downside to this approach.
They could grab magic initiate, or multiclass, to pick up cure wounds to heal more and it would actually work without having to ignore a part of the spell, or the RAI
Oh yeah, I fully understand that so many was to do something like this are better. You’re using a 6th level spell, a 3rd level spell, a 1,500 gp component, and using up your contingent spell on this just to get back ½ of 3d10 on a hit.
This is about one of the least broken healing tricks you can do in 5e, but so many people are going out of their way picking at minutia or saying “Here’s how I’d houserule this to stop that trick” (essentially admitting it works fine without DM fiat to counteract it) without considering that life transference is infinitely better and also fails to exclude yourself a viable target for healing. Or just polymorphing yourself, or putting yourself in a resilient spherebefore you take the damage is perfectly valid, strictly better and still an utter waste of a contingency.
Just FYI though, this is what being creative with spells actually looks like. Coming up with a weird unforseen non-RAI use-case and implementing it within the bounds of the actual words of the spells. Not reading the name of the spell and saying, “I create water inside his lungs, instantly drowning him. (Pls don’t look up suffocation rulez. thx” or “I heat the metal calcium in his bones, lol.”
All that aside, it looks like my pot stirring was a bit more successful this go ‘round. If I got some people to sign up to argue with me and migrate away from that site I used to use before it became enshittified beyond human tolerance, my purpose was served.
DM says "That's clearly not how the spell was ever intended to work and your explanation defies anything resembling common sense. You take two death save fails and lose the spell. Fuck off."
Doesn’t say “exclusively from others”. Without casting this spell you couldn’t do that normally.
It also doesn't say that you cannot fly yet that doesn't mean it gives you a flying speed. The spell only does what it says it does: It makes you able to siphon life force from others. In any situation not explicitly mentioned in the spell you are still bound by all of the other game rules and as far as I know there is no rule that'd say you can go siphoning life forces from anything without an effect explicitly stating that you can do so.
The D&D rules do not run on a principle of "you can do absolutely anything except for what is explicitly forbidden".
No qualifiers about the target having to be a creature other than you. It just has to be a creature within your reach.
You do not get to cherrypick which sentences of the spell description you read and follow. It does quite explicitly state "can siphon life force from others to heal your wounds". There is no concept of flavor text in 5e, every word of a spell description is rules.