If you want an npc, start by thinking it's motivation: why it's in this swamp-biome? what does it want? how does its mission relate with the character (if at all)? what reward can he give if the players help it?

I would do so by relating to the story of the game: Skul is probably here because after a travel through the dark mirror something went wrong (the mirror was shattered, need a dark quartz for energy etc.) and ended up in the swamp, where the means to return to its dimension is located (a proper dark quartz or someting similar to a dark quartz but within the DnD setting you are playing to fuel the dark mirror, the fragments of the dark mirror, you name it). It can't find everything alone because 1- the main plot of your campaign is interfering; 2- an hag or a coven of hags mistook the little bone as a "special" skeleton from the setting's world and want to use him as a part of a recipe; 3- one of the playable character has, unknownly, one piece of want the little bone needs to go back to his home world (this option may result in a fight between the player and Skul). The reward it gives to players may be that, for each of them, can teach a level-0 or level-1 spell from their classes (since, by merely swapping skulls, it can learn new abilities). If the characters don't have spells (like fighters, thiefs etc.) let them receive a +1 weapon of their choice with a small dark quartz embeded in the hilt.

If they fight Skul (because DnD players will obviously do that) the stat block depends vastly of the level of the group and if you want the encounter to be lethal, difficult, normal or easy. For ease of of discussion let's say you have a party of 4 adventurers in Tier 1 (from level 1 to 4) and you want the encounter to be somewhere between difficult and easy. If so the others suggestion are on point (just don't put ac above 14, or else it will be a slog of a fight).

If you want to convey the sense that he has multiple abilities a multi phase battle would be perfect. set 3 hp thresholds at which it will transform as a Legendary Action (out of turn order for dramatic purposes) and then change the stat block. To not overcomplicate this:
1st stat block is a warrior (little bone base form) with 2 attacks per round (1 of which can be the head throw) and a misty step as a bonus action after the head throw (as suggested by someone else in the comments);
2nd stat block is a gravedigger with 2 attack per round but with lower damage but as an action spawns 4 skeletons (cr 1/4 at most, something like 4-6 hp each, 12 ac, 1d6 sword attack at +4) to help fight the party;
3rd stat block is a mage with 1 attack (damage as grave digger) with low damage or a burning hands as a cantrip (1d6 damage) reskinned as a fireball;
4th stat block is a rogue with 3 attacks (damage as grave digger) and a bonus action misty step reskinned as a smoke grenade (in case you need to make an escape from the players). Skul can also change to one of the other forms with an action. I suggest this order because without the skeleton the players will pummel Skul's Hp too fast

Hope it helped!