The Death Of Balreth

Objective 1: Summon Blaze and Ragebinder
This part is the same as the objective in "Rending Flames" quest. Blaze’s Ritual Braziers are just south of the location where you’ve picked the "The Death of Balreth" quest.

Objective 2: Talk to Mavos Siloreth
Mavos is satisfied with your work and he opens up a portal behind him. At this point you might want to invite some other people around the portal to join you for this quest.

Objective 3:Enter Ash Mountain
This is where the hardest part of
"The Death of Balreth" takes place.

Objective: Defeat Balreth
Balreth has three important characteristics you should be aware of. The First move: Balreth patrols from one lava pool to another. When he reaches it, he starts regaining his health. The Second Move: While he is on a platform where he patrols, he summons an add and casts a fire trail that moves away from him.

How to deal with him alone? At the same platform where you fight vs Balreth there is NPC called Garyn Indoril. This npc is your ticket to a winning the fight. If you stand close to him he revives your health, magicka and stamina. Another helpful device is the Storm Talisman. This medium time cooldown ability allows you to stun Balreth for a short period of time.

How to defeat Balreth? Damage, damage, damage. This should be followed by a wise usage of Storm Talisman. Sometimes this Storm Talisman stops Balreth from regaining his health in lava pools. Nevertheless, use it while he is still on the platform to slow his movement towards the other lava pool. Be close to Garyn Indoril for faster regeneration of your health and mana.

Objective 4: Talk to Mavos Siloreth
When you defeat Balreth, Mavos Siloreth shows up on the same platform.

Objective 5: Leave Ash Mountain and Talk to Walks-in-Ash
Portal that teleports you out of Ash Mountain, takes you to your last quest objective NPC.

This mission rewards achievement "The Death of Balreth" at the end of successful completion.