Grand Fin-ale
Summon Elder Murk-Eye using a Mrgrglhjorn, then defeat him and Harjatan in Tomb of Sargeras on Normal difficulty or higher.
ACHIEVEMENT MAP
Grand Fin-ale is a 10-point achievement earned by summoning Elder Murk-Eye with the 
Mrgrglhjorn toy during the Harjatan encounter in Tomb of Sargeras on Normal difficulty or higher, then defeating both Elder Murk-Eye and Harjatan to earn the Murloc's Wind Chimes decor reward.
Guide awaiting review
Obtain Mrgrglhjorn First
At least one person in the raid must have the Mrgrglhjorn toy in their collection, which is obtained by completing the No Shellfish Endeavor achievement.
Enter Tomb of Sargeras
Set the raid to Normal difficulty or higher and clear the trash leading to Harjatan before engaging him.
Engage Harjatan, Then Summon
Use a mild stun to engage Harjatan first, then immediately use the Mrgrglhjorn toy to summon Elder Murk-Eye — do not use the toy before the fight begins, or you will trigger a 30-minute cooldown.
Kill Elder Murk-Eye First
Target and kill Elder Murk-Eye before turning your attention to Harjatan.
Defeat Harjatan
After Elder Murk-Eye is dead, kill Harjatan to complete the achievement.
Tips
If Elder Murk-Eye fails to spawn, try the encounter on a lower item level character (such as a level 70 with around 405 ilvl gear) to ensure the toy can be used during the active combat window.
See the video guides for a visual walkthrough of the toy timing during the Harjatan encounter.