For anyone interested, the sword transmog is the Blazing Demonhilt Sword, which drops from Hellfire High Council in Mythic Hellfire Citadel. Mythic is green flame, Heroic is blue flame, and Normal is orange.
Update: on July 4, 2024, after so many runs that I’ve lost count, the off-hand finally dropped. Unfortunately I wasn’t able to record the run, but I’ve finally equipped both and unlocked the achievement Warglaives of Azzinoth. Now I just need to wait until end of August to run the Timewalking raid and get the cosmetic achievement unlock.
Proof! It only took a couple of years to get both, but the chain skip saved a lot of time.
