A computer made out of the undead at the whim of a mad lich. It originated when someone on /tg/ realized that since undead skeletons and zombies could be given simple commands (e.g. Attack anyone that enters this room except myself), they could be ordered to react to the actions of other skeletons and zombies
(e.g. If one of those two skeletons raises their hand, raise yours, but not if they both raise their hands. This replicates the functionality of an exclusive-OR gate).
WTF is this?
OK, first Computer science for dummies in six seconds.
A computer works by storing information in a unit of information called a bit. A bit is a simple positive or negative status, yes or no, only two options which is why computers are said to use binary language. Disks work by having thousands of dips and bumps in their surface that are read by a laser. Bits are then grouped into more complex bytes which roughly equal a single laser worth of information. It takes roughly 8 bits of information (eight 'yes' or 'no') to make a byte and bytes are typically the smallest unit of information a computer works with. a byte can represent 256 different values. Bits also use the binary system for big things; a Kilobyte is 210 bytes big, or 1024 bytes.
Deep Rot is the same way only instead of utilizing circuit boards (your computer), vacuum tubes (ENIAC) or gears(the Antikythera mechanism), to compute the simple yes-no operations, it uses skeletons.
The concept is simple. A skeleton is stupid, but smart enough to recognize a simple a yes/no situation. Imagine a nine skeleton set up. Eight skeletons are told to raise or lower the left arm in a pattern. Down, up, up, down, up ,down, up, up; Or, in binary, 01101011.
Skeleton nine is watching the first eight, and recognize that "01101011" equals the letter K, this simple ability is the base of information storage, and from that computing.
Additionally: a single skeleton can hold more than a single bit of information. More than just 'raise left arm' you can store as many as ten bits of information on a single skeleton since you're using all the fingers. Even more if you consider the head as an 11th bit, more if you count arm position. The only limit to how much information a skeleton can hold is how many states of information the Skeleton receiving the information can understand, and how complex the programmer can make it.
For the sake of argument we'll say a Skeleton can hold two bits of information as a simple baseline; so four skeletons can make a byte. using that as our base we can say that it would take 360 skeletons to store a typical line of text from a book, 160000 to store a page of a novel, and a mind-boggling 4,194,304 to store a three-minute song.
Your average flash drive has 8GB, to replicate that storage in skeletons you need four billion. There are roughly seven billion people on this planet (as of 2020), so you would need to kill four out of every seven people, on the whole planet in order to make a working Skeleton Computer FLASH DRIVE.
In order to get a computer that can 'compute' you need billions more skeletons in order to have enough 'bits' that it can make decisions.
Jaded viewpoints aside, a capable computer in the traditional 1950's sense could be established with a few thousand skels. But be prepared to make an eloquent argument to your dm...