Grungni was born as one of the first three dwarfs alongside Grimnir and Valaya. He was the father of the entire Dwarf race, and was the one who taught them how to use and shape the metal in the earth.
Age of Sigmar
He returned in Age of Sigmar alongside Grimnir, being found chained to a mountain. He was recruited into Sigmar's pantheon of Order, and helped in the creation of the Stormcast. Other than sending a group to recover the Eight Lamentations (a series of deadly weapons), he hasnt really been doing much, probably on account of grumbling about how every piece of equipment issued is barely adequate and far too shoddy.
In recent lore he has been revealed to have been on a self imposed exile in shame, after Chaos left his dwarves without a home, many either dead or joined chaos as members of the Iron Golems or the Legions of Azgorh, but recently he has resurfaced doing two major things, one is selecting a DnD style party to help him recover the eight lamentations, chaos weapons that can cause absolute destruction, created by.....you guessed it Khorne himself, also it is made out of the body parts of Khaine (except his heart, Morathi has it), and the second thing he has been up to is still a secret but all hints point to a third dwarven faction that will "reunite the dispossessed dwarves" A.K.A a more "traditional" dwarf faction that, much like the Lumineth Realm-Lords, will replicate the Dwarven Realms of the Old World, so Fantasy Dwarves fans rejoice, while Cities of Sigmar fans, be afraid, as this usually is followed by Nuking the factions Fantasy models form the Cities of Sigmar lineup (see High elves when the Lumineth were released), this has already been foreshadowed as Grungni has explicitly said that he will reunite the dispossessed.
He may or may not have made an appearance in Broken Realms: Be'lakor as just as the first Chaos Prince is about to attack the Khadron Overlord airship fleet he abruptly backs off when he senses divine power apparently radiating from one of the Duardin troops, though it's still unclear if this was an avatar of Grungni or the White Dwarf.
Grugni definitely appears in the final book of the Broken Realms series, Broken Realms: Kragnos. Arriving in the throne room of the Ghurish city of Excelsis after the battle for it is over he stops the bickering of the Celestant Prime and Morathi. Grungni points out that while Morathi's actions throughout the saga were dickish, if she hadn't ascended to godhood then Kragnos and his forces would have obliterated Excelsis so they should give her a pass for her earlier actions and just move on to focus on the threats of Chaos and the newly galvanized forces of destruction.
The preview for the new 3rd edition explains that he is back to help Sigmar create the new Thunderstrike armor the Stormcast will be wearing. It apparently has a stronger connection to the realm of Azyr and thus should hopefully allow them to return to Azyr upon their deaths despite the Storms of Chaos Be'lakor has gone and unleashed on the realms. Only time will tell!
For what it's worth, the Kharadron Overlords update article in White Dwarf #465 states the Grungni has returned and this has led to a major shakeup in Kharadron society - Grungni wants to reconcile, but accepts that he must wait for them to come to him first, and the Kharadrons are largely hesitant to do so. His potential return is also mentioned in the Gotrek novel "Gitslayer".
- Grimnir, his brother
|The Grand Pantheon of Age of Sigmar|
|Alarielle - Gorkamorka (Gork + Mork) - Grimnir - Grungni - Malerion - Nagash - Sigmar - Teclis - Tyrion|