Despite their allegiance to Nurgle, the Glottkin are actually Imperial by blood; the sons of a Nordlander father, Ollos Glotts, and his wife, a Life mage named Ethra Greenblood. The Glotts came to Norsca on a secret mission; by living peacefully alongside the Norscans of the fjords of the Sea of Claws, showing them the healing arts of the Wind of Life and more advanced methods for coaxing successful crops, they hoped to slowly break the chains of hatred that compelled the incessant cycle of raids and retaliations.
For some time, the two were welcome, becoming quite influential, but a jealous Norscan hag cut Ethra's finger with a cursed rusty knife during her pregnancy with her sons. Unable to break the hag's curse herself, Ethra pleaded with the gods to spare her children, and Nurgle took pity, curing the infection and allowing for the birth of three healthy boys; Otto, Ethrac and Ghurek. Each was healthy and hearty, the only sign of their future fate a birthmark with three lobes that each bore on their body.
Their efforts were doomed when a Nordlander force came to Fjordlingtribe land in 2506; the Glotts took up arms to defend their friends and neighbors, and the brothers saw their parents murdered by the Imperials. From that moment on, their souls belonged to Nurgle.
Ghurek devolved into a monstrous beast, an eating machine capable of swallowing men whole and then shitting them out as grotesque Chaos Spawn Oh ShiARGH!, to the point his own brothers took to calling him Ghurk and using him as a steed. Ethrac became a powerful sorcerer of Nurgle, using the black arts to slay and murder. And Otto turned his scythe into a lethal killing tool as a champion of Nurgle.
They were chosen by Archaon to lead his second invasion during the End Times, allied to the Maggoth Riders and to Gutrot Spume. They destroyed the cities of Mareinburg and Carroburg, and laid siege to Altdorf. Otto fought Vlad Von Carstein, and almost lost if Vlad didn't make the crucial mistake of wielding a sword that let you automatically drink the blood of your enemies against someone so filthy that their blood is probably brown. Otto did manage to briefly kill Karl Franz, until he came back to life divinely empowered and kicked their asses. They only escaped death by changing into fly swarms, because they figured if Karl Franz can pull a bullshit deus ex machina out of his ass so can they. Now the triplets are trapped in jars in Nurgle's attic, for Nurgle is very displeased with them...
The Three returned in Age of Sigmar however (the Maggotkin Codex explained that while Nurgle may punish his minions for failing, he always eventually forgives them), with Otto being the commanding general of Nurgle's invasion of the Realm of Life alongside his brothers. In these stories, the Glottkin are shown to be arrogant pinnacles of what champions of the Plague God aspire to be, often resulting in them arguing with one another, though they still retain a strong brotherly love to each other.
They also have fantastic south-west England accents.
On the tabletop, the Glottkin fight as a single Special Character Monster, since Otto and Ethrac always ride little brother Ghurk into the fray. They cost 810 points and use up a Lords slot. They have Movement 6, Weapon Skill 6, Ballistic Skill 3, Strength 6, Toughness 6, an insane 12 Wounds, Initiative 1, 5 Attacks and Leadership 10. They have the Nurgle's Rot mutation, and the common special rules Eye of the Gods, Impact Hits (D3), Mark of Nurgle, Poisoned Attacks, Regeneration and Terror. They also have the unique special rules of Ethrac Glott (they're a level 4 wizard using the Lore of Nurgle), Ghurk Glott (chose one attack per round; this one is S10 and Multiple Wounds (D6)), Otto Glott (gain +D6 attacks at the start of each Close Combat Phase), Lords of Nurgle (Glottkin and Nurgle-marked units within 12" re-roll failed charges, Glottkin always get the Aura of Chaos reward when rolling on the Eye of the Gods), and Pestilential Torrent (Strength 3 Breath Weapon that ignores armor, can only be used in close combat).
While this all sounds impressive, the issue with the Glottkin is that unlike the other behemoth model (Nagash), they get no real bonuses to boost themselves or their forces, for example Nagash has triple the range and summon costs from the Lore of Undeath, but the Glottkin gain no such bonuses for the Lore of Nurgle or the new "Summon Infernal Legion" spell (it should be noted that the Summon Infernal Legion spell is specific to the "Chaos Ascendant" special scenario ruleset, and is not a universal new rule- you only get it if you are playing one of the special scenarios or your opponent otherwise agrees to it). Taking the trio means that they'll either need to be in constant combat in order to make back their points' worth, or you're going to be casting that summon spell as often as possible (if not both "Summon Undead Legion" and "Ryze - The Grave Call" to get two summon spells out of them), but again, unlike Nagash they have no protection at all against miscasts and at their cost means you don't want to take that risk unless you're badly losing the game. Lastly as a monster, they cannot join units and so can be picked out by mass archer spam or siege machines.
In Age of Sigmar, Glottkin is one of the tankiest models in the game. With a whopping 18 wounds, healing d3 wounds per round, it is unlikely to be killed by the end of most battles. Aside from the usual array of powerful attacks, Glottkin more than pays for himself through his utility: As a general, his command ability gives all units with keyword: NURGLE an extra attack FOR EACH MELEE WEAPON. This isn't just Rotbringers; any unit with the NURGLE keyword is affected, including Slaves to Darkness, Daemons of Nurgle, Pestilens Skaven, the Everchosen(making him great as a sub-general under Archeon), and even the Forge World exclusive Tamurkhan's Horde. Try sending a hundred rats at the enemy with 5 attacks a piece, or give your tarpit teeth by using a unit or two of plaguebearers. Ethrac also gives the powerful spell Fleshy Abundance, which temporarily gives a unit an extra wound, essentially doubling the endurance of one-wound horde models like Plaguebearers and Maurauders.