This boss is technically optional and I will list the requirements below:
The story line will task you with obtaining the Guardian's Heart. This is achieved by killing the Ixillis XV boss. Once defeated you can either give it to the Undying King or to the Iskal Queen.
Giving it to the Iskal Queen will cause the Undying King to initiate a fight when you next speak to him. If you give the heart to the King you will get a separate achievement, and can do the opposite on another playthrough if you wish.
The fight itself:
As with pretty much all boss fights so far the adds are where the fights usually fail. Once this fight starts you will have 4 sentry looking dudes that you need to kill, and once done the boss will spawn blue symbols that shoot you for decent damage. Focus on taking these out before attacking the boss.
On the left an right side of the room there are capsules and it seems the adds spawn from here, so keep an eye on that. After you damage him enough he will slowly walk back to his stating position and this is where it can get crazy. The natural instinct here is to attack him, but, this is when he spawns a lot of adds which need to be taken care of. Once they are gone focus damage on the boss.
Not sure if this is a glitch or by design, but once you have depleted his health bar once, he gets back up with half health. Didn't appear to be any difference with his attacks, though.
For weapons I had a Submachine Gun +10 and an Assault Rifle + 10 using the Hunter's Mark and Swarm weapons mods. The first makes it really easy to keep track of boss and adds, and the latter really helps distract the adds whilst you heal/use ammo, etc.