Update: This bug has been fixed on patch 2.0.5.
Here’s a current bug with the Immortal King Set when using Call of the Ancients – Together as One rune. What this does is basically give you a free 50% reduction in damage taken. You can use this with any build, you just need to have access to the Immortal King’s 4pc bonus.
Basically put on the IK set, cast Call of the Ancients with Together as One rune and then put your other gear back on. The ancients will disappear but the buff will remain. You can also wait for the CotA cooldown to reset and you can even switch that skill out for whatever else you want and the buff will stay until you make a new game. Hopefully Blizzard will get this bug fixed very soon.