Maven and unkillable Mob

Just beat the maven but cant choose another favorite map nor did i get a voidstone, just a crescent splinter.
Instead the defeat 10 Tier 14+ maps progress starts again which i already did (collected 3 invitations).
What do i have to do to get that voidstone?



Also i encountered an unkillable mob which is strange because he was vulnerable to fire which i deal. The modifiers give me no indication why i couldnt deal damage to him. Stood minutes in the mob, not even shields went down.
What was that?

You didn't beat the maven. Sounds like you won the 10 boss fight. To fight the maven (and get the voidstone), you need to collect 10 maven writ fragments to form a maven writ, then put that into the map device. Be warned, the maven is much harder than that 10-boss fight you just won.

OK, now i understand.

You can also increase the amount of splinters dropped by rolling the invitations. With high enough quant and running the atlas passives that causes invitation modifiers to have 100% increased effect you can easily get 4-5 splinters per run at the cost of the invitations being much harder with all the added mods.

You need 10 of those to make the key for maven fight.

As for why that mob was invincible, looking at the minimap there was a shrine right behind the door where you fought the guy, that was most likely a divine shrine who makes him invulnerable. Either capture the shrine by clicking on it or lure the monster away and kill it once it's out of the shrines range.

Thank you Sirs