Tamurkhan the Maggot Lord is a Chaos Lord dedicated to Nurgle in Warhammer Fantasy Battle. He is the titular character of Forge World's Tamurkhan: Throne of Chaos campaign book, and claims to be one of the four sons of the Great Kurgan, a Warrior who at one point united all of the tribes and forged a mighty empire. This was well before the time of Sigmar and even Ulric, which would mean Tamurkhan is FUCKING OLD; this could explain his bad habit of sitting around not doing shit for an entire month, while his armies grow bored and his enemies prepare their armies. In fact, if an assassin didn't try to kill him, it's possible that he would have stayed meditating forever.
Tamurkhan's moniker is no mere affectation; thanks to a unique gift from Papa Nurgle, his true form is a pestilent, slime-dripping maggot-thing the size of a human child, which can (and must for him to survive) bore into living victims and devour their guts, then take control of the corpse as a secondary skin. Through this manner, he has achieved a twisted immortality, allowing him to fight inside a corpse-host, then attack and possess his opponent (or a living ally, if shot down from afar) to keep on fighting when it is destroyed.
During the events of Throne of Chaos, Tamurkhan wore three corpse hosts. He participated in the battle royale that was going on between the champions of all four gods while wearing the body of some random Chaos Lord. He won the battle and forced them to serve him by single-handedly by possessing Sargath the Vain, a Chaos Champion of Slaanesh, after Sargath wrecked his previous body. Interestingly, he retains any gifts or abilities his hosts have for a period of time before they rot away (and possibly even after they should rot away), Sargath's Slaanesh gifted voice being one of the reasons he gained such a massive horde. During a successful attempt to get some Chaos Dwarfs on his side, Tamurkhan was ambushed by an Ogre tribe and took over the body of Ogre Tyrant Karaka Breakmountain after Karaka quite literally ripped Sargath's body apart - gaining Ogre auxiliaries in his army in the process. In his Sargath and Tyrant forms (and presumably countless more before) he rode Bubebolos, greatest of the Toad Dragons.
Soon he invaded the Empire to claim the Throne of Chaos by claiming Nuln for Chaos. Originally his plan was to launch a surprise attack by using the Chaos Dwarfs to reduce its walls to ruin, as the defenders wouldn't be expecting chaos forces to have good siege equipment, but he accidentally attacked its sister city and alerted Nuln to what he was going to do when he reduced it to ruin. Due to this Nuln was as prepared as it could be when he came after it and he wasn't able to pull off the same trick twice.
The battle at the city wall didn't quite go in his favour, so he instead chose to rely on daemons to save the day. Doing so involved sacrificing many people as well as his Toad Dragon, but even this wasn't enough as he fell at last when he tried to take the body of Theodore Bruckner during a duel between the two, only for the Baleflame Amulet gifted to him by Magisterix Elspeth von Draken to perform its deadly task: immolating both slain Imperial champion and Chaos-get abomination in one fell surge of arcane flames. Thus perished Tamurkhan, and ended his pursuit of the Throne of Chaos.
Such was Tamurkhan's loyalty that Nurgle planted a piece of his soul within the Realm of Chaos, allowing him to be reborn in the Mortal Realms. Which is Forge World's hand-wave for why you can still use him in Age of Sigmar. Between him, the Glottkin, Gutrot Spume, the Maggoth Riders, and Festus the Leechlord, it seems Nurgle really likes bringing back his champions from the World-that-Was. Which makes sense; Khorne and the Horned Rat don't forgive failure, Tzeentch is too fickle to be consistently forgiving and while Slaanesh may love his/her followers he/she does so in all the wrong ways.
On The Tabletop
Tamurkhan along with Bubebolos is an absolute beast, and his points costs reflect it. In his Sargath body he's surprisingly weak, gaining -1 Movement, -1 Weapon Skill, -2 Initiative, -1 Attack, compared to a regular Chaos Lord, but this doesn't matter as you can (and probably should) upgrade him to his Tyrant body where he gains +1 Movement, -1 Weapon Skill, +1 Toughness, +3 Wounds, -4 Initiative compared to a regular Chaos Lord. He also MUST be the General if he's in your army and he has different weapons depending on which body he's in. Sargath has the better weapon with Armour Piercing and Multiple Wounds (2), and in his Tyrant Body he gains a Great Weapon that forces the survivor to take a Toughness test on an unsaved wound, suffering a -1 to their Toughness when they fail - which is fairly pointless since every high Toughness model he's up against is likely to die before they fail, and every low Toughness model could already be wounded by him and his Toad Dragon on a 2+ anyway (since he's effectively Strength 7, and his Toad Dragon is Strength 8).
He also has Mark of Nurgle, which is a given, and the "Will of the Gods" special rule, which is probably supposed to be The Will of Chaos since this came out before the Warriors of Chaos got their 8th edition book. The Will of Chaos means he can re-roll panic checks, but Will of the Gods is also the rule that allows a model to join a unit, even if they cannot thanks to rules like being on a monster, so make your own judgement and talk it over with who you're playing against first, since Forgeworld didn't add it to their FAQ or fix their PDF.
Thankfully for his Sargath body, Tamurkhan's greatest rule doesn't rely on him surviving very long, and considering how large of a target he it he'll be given a lot of firepower from any sort of siege machine. His possessor rule means that if he's killed in close combat, he'll immediately try to possess his killer; otherwise he'll posses the nearest Infantry or Monstrous Infantry model within 6" (this can be your own model too, so keep your characters away from him). In this case a dice is rolled for both models and added to their weapon skill; if Tamurkhan has the higher score, he immediately becomes the model he tried possessing, inflicting -1 Movement, +1 Toughness, +1 Wounds, -1 Initiative on whichever profile they had, while keeping their Strength and Attacks and changing their Weapon Skill to 7, their Ballistic Skill to 3, and their Leadership to 9. He also loses whatever equipment he had (meaning his Chaos Armour and whatever weapon he had) and starts using the equipment of the model he possessed, so if you're lucky he'll get a massive boost in damage output if he's killed by somebody like an Ogre Tyrant (oh wait), or another Chaos Lord.
All of that is extremely underwhelming for 645/835 points, but thankfully he's also got Bubebolos! Compared to a regular Chaos Dragon, it cannot fly but gets +2 Movement, -1 Weapon Skill, +2 Strength, +1 Toughness, +4 wounds(!), -1 Initiative, -2 Attacks and -1 Leadership. His 4 attacks might seem disappointing until you remember that he inflicts 2D6 attacks for Thunderstomp, and has the Tongue Lash rule, easily his best rule as it gives him an ASF poisoned attack at Strength 4, and should the victim survive the attack (technically it doesn't even need to hit the target) they suffer -1 to hit. This combos excellently with the Mark of Nurgle both Bubebolos and Tamurkhan have, meaning anyone the tongue attack is directed at can only hit Bubebolos and Tamurkhan on a 5+ at absolute best (if they have WS 6 for Bubebolos or WS 8 for Tamurkhan), and at all other times can only hit the pair on a 6+!
Bubebolos also nicked Archaon's special rule, but did it better, with no attacks being able to wound him on better than a 4+, and no attacks weaker than Strength 4 can even hurt him (just in case you wanted to say "Fuck You" to those elves). He one ups the regular dragon with a better Breath attack which has an odd way of wounding: the enemy must take a Toughness test at -1 to their Toughness, and if they fail they suffer D3 wounds with no armour save allowed, giving you another ability to fuck over anybody who's Strength 3 and/or Toughness 3. Lastly, you might think Initiative 2 might leave him vulnerable for Initiative Test or die spells, but not so since if he's hit by any spell that would instantly kill him, he instead loses D6 wounds - given that he has 10, it means he'll still live quite a long while.
If there's one thing that can royally fuck over Tamurkhan, though, it's the Eye of the Gods table. Turning him into anything really fucking sucks since you lose the possession attack, and since he's only armed with Chaos Armour and a Magic Weapon, even as a Daemon Prince he won't live very long.