Q:

The plausibility of a pack-hunting carrion-eating snake

I was thinking of a hypothetical species of snakes that join together for protection called Ladôni, due to their habit of mobilising in massive beds which make them resemble a large multi-headed snake. Ladôni often feeds on dead carcasses although, they'll sometimes hunt young, old, sick or injured prey by attacking it en masse with saliva anticoagulants and burrowing into the prey to eat it alive from the inside.
Could such a creature exist and if so, what sort of adaptions would it require?

A:

Snakes ingest their prey whole, usually head first. After digestion they usually regurgitate the undigested parts, like horns, hair and the like. They lack the ability of taking chops from a prey.
As such it is impossible for a snake to eat a prey from the inside.
Moreover, swallowing a creature whole can be done only if it is dead: no matter how weak it is, you don't want it to fight around while halfway down your guts, tearing them open. 
Additionally, per Nyakouai's comment, a snake which has just swallowed a prey might not be really effective at hunting for a while, so a pack of snakes means one get fed for each prey taken down and is then virtually useless to the pack, so not a great survival tactic as a whole. I have once seen a snake which just had swallowed a chicken egg: the poor beast was so incapable of running away from me with the egg enlarging its body that it preferred regurgitating it, still whole, in order to reach safety. After the whole process it was so exhausted that I could grab it and take it to a safer location.

A:

You could probably create such a species by taking inspiration from several different reptiles and other animals.
Blindworms look deceptively like snakes, but they actually are legless lizzards. Due to differences in evolution, it's plausible they would have eating habits that differ from that of snakes.
It's rare, but not unheard of snakes piling together in earth pits to preserve warmth in areas where the nights can become dangerously cold. Have them evolve in a habitat where small prey gradually went extinct over several generations and you have a plausible reason for the snake-pile seaking out carcasses as a pack.
To create a truely multi-headed snake-pile that moves as one into the direction of the next carcass, you could improve their ability to detect smallest traces of the smell of decay. Vultures are known to accumulate around carcasses despite having only limited social behavior, simply because all birds in a certain area follow the same scent trail. If your snake-pack sleeps in one place, it's plausible that multiple members perceive the scent trail of the next carcass and other members simply follow because this behavior usually yields a meal.
Several species of fish and birds developed movement patterns that keep shoals and flocks together and moving as one unit. The snake-pack doesn't even need very sophisticated swarm-behavior, because they can glide over each other without causing them to crash or being eaten. All they need is a disposition to avoid the direction where none of their kin can be seen and viola! instant snake swarm.
Next mix in some Aligator. 

Alligators consume food that cannot be eaten in one bite by allowing it to rot, or by biting and then spinning or convulsing wildly until bite-sized chunks are torn off. This is referred to as a "death roll".

You could easily envision a writhering, convulsing mass of snake-like bodies ripping tiny chunks out of a carcass.
The only thing I cannot quite envision is the snake-pack hunting down live animals. It's far too dangerous for non-poisenous reptiles to hunt prey bigger than them. It would require a tremendous combined effort of the pack that probably 
necessitate a level of intelligence far above what reptiles are capable of. Poisenous snakes, on the other hand, have no reason to hunt together. 

A:

Do you REALLY mean pack-hunting, or do you just mean schooling/swarming? Pack-hunters (e.g. Lions, Hyenas, Humans) are intelligent, and cooperative, and voluntarily share the benefits of their kills. They're also almost exclusively apex predators. The only reptiles that do this are crocodiles, and only VERY rarely. 
What it sounds like you're describing is really more of a non-aquatic Hagfish. 

... hagfish can feed upon and often even enter and eviscerate the
  bodies of dead and dying/injured sea creatures much larger than
  themselves. They are known to devour their prey from the inside.[44]
  Hagfish have the ability to absorb dissolved organic matter across the
  skin and gill, which may be an adaptation to a scavenging lifestyle,
  allowing them to maximize sporadic opportunities for feeding.

I can't think of any reason this behavior pattern wouldn't work just as well for a snake as it does for a fish. There are certainly snakes that hunt in swarms, although again not in the cooperative way that defines pack-hunting. 
(pretty sure a segment from Blue Planet II about these guys is what @Shadowzee was thinking of.) 
To make a land-dwelling equivalent of this, I think you'd just need to tailor your environment such that there's adequate food sources, and remove the potential competition from mammalian predators. 

