So I thought it would be fun to go through all the different times Morghur the Shadowgave "died". So here's a list:
1. Ariel first senses Morghur's corruption and sends several scouts to check him out. None of them come back. She decides if she wants something done right, she would have to do it herself, so she spirit projects herself to where Morghur was and laughs at how pathetic of a creature he was. She released cleansing flame on him and left, thinking the job was done. Of course, she underestimated his regeneration, so it didn't actually "kill" him. This starts Morghur's obsession with Ariel and Athel Loren.
2. Two centuries later, Morghur returns, attacking Athel Loren with a large warherd. The Asrai didn't notice him because they were too buys engaging in blood sport with their "Wild Hunts", not only were they slaughtering the humans and greenskins of Bretonnia, but they delighted in manipulating them to fight each other. Morghur charged into Athel Loren and overwhelmed their forces until he was captured and held down by the Treeman Ancient Coedill. Ariel decided she wouldn't make the same mistake again and summoned the power of the Forest to obliterate Morghur. Unfortunately Morghur's corruption transfers to a portion of the Forest and the Witherhold is never the same again, becoming a corrupted patch of forest ever since. Coedill was also most likely corrupted, as he betrayed and attacked the Elves soon afterwards and was sealed in the Wildwood as a result.
3. Morghur is reborn a while later and instead of going for Athel Loren, he attacks Silverspire in Bretonnian land. Ariel sense that it's a place of great importance and its water feeds Athel Loren, so Orion goes out to hunt Morghur for her with his Wild Hunt. He slaughters a bunch of humans on his path but who cares they're human, he find Morghur and tears him limb from limb. He also senses a familiar presence in the Silverspire...
4. Morghur is reborn again a while later and attacks Silverspire again. This time, Orion didn't lead the Asrai host, which is a good thing because Bretonnia is actually a thing by this point and the Asrai actually had to cooperate with Gilles le Briton's army to defeat Morghur. Gilles' son comes over to the Asrai to form a treaty with them for peaceful contact. Ariel agrees because she believes they'll be a decent shield againt Morghur, Orion is sulky and tells her he won't stop hunting them down during the Wild Hunt. She smiles and tells him he can still do that, they're just humans after all. Lady of the Lake worship spreads around Bretonnia. To those not catching the hints, she's what's inside Silverspire.
5. A lot of things happen between the previous incarnation of Morghur and this one. To make a long story short, Ariel gets super corrupted as she starts pursuing Dark Magic due to a tragic (and near comical) series of events that went in just the right way resulting in Ariel getting Dark Magic Lore from Morathi. She grows more vengeful and unstable and decides she'll consume Morghur to grow more powerful and avenge all the wrongs done to her and Athel Loren. She successfully captures Morghur, brings him to the Oak of Ages and is about to consume him when Durthu, one of the last remaining Treeman Ancients, senses what's happening. He rushes to the Oak of Ages and kills Morghur before the ritual finishes. Ariel gets mad at him and banishes him. She is later purified when Adanhu, another Treeman Ancient, sacrifices himself by absorbing the taint from her soul. He dies and Ariel finds out how horrible the things she did were, so she seals herself in the Oak.
6. Ariel leave the Oak three centuries after she sealed herself in there when she senses Morghur is back. The entirety of Athel Loren unites to defeat Morghur's Beastherd in the Forest of Arden around 2000 IC, with Orion and his Wild Hunt, Ariel, the Sisters of Twilight and more. The one to deal the finishing blow, with an arrow straight through the eye to the skull, is none other than Scarloc.
7. The Battle of Arden. Already discussed at length. In canon Glade Lord Araloth is the one who kills him with sap from the Oak of Ages, don't know if it's the same in DL.
So, final count and list of those who "killed" Morghur:
1.Mage Queen Ariel
2.Treeman Ancient Coedill and Mage Queen Ariel
3.Orion and the Wild Hunt
4.Gilles le Briton's host and Athel Loren
5.Treeman Ancient Durthu (although Morghur was bound by Ariel at the time)
6.Athel Loren's forces. Finishing shot by Scarloc
7.Glade Lord Araloth and his forces from Athel Loren. Bretonnia and Asur get an assist credit in DL
