Flexible Mythic Slot
When the one-boss raid Sporefall was released in Season 1, some players noticed that one of their Great Vault slots was listed as Flexible Mythic. With the Tidebound Grotto Lair in Season 2 also having Mythic Flex difficulty, players have noticed the "Mythic (Flexible Raiding)" message in their Great Vault again.
Mythic Nymrissa Wavecaller seems to count as a Mythic boss in the Great Vault, but at a lower priority than a Mythic Venomous Abyss boss. For example, if you kill 5 Venomous Abyss bosses and Mythic Nymrissa, the final slot will say "Mythic (Flexible Raiding)". However, if you kill a 6th boss, then the Mythic Nymrissa item gets pushed out, and all 6 slots are Mythic.
Guaranteed Nymrissa Loot
Draco, Geek, Umbrica, and others from the Raid Leader Discord looked at some Great Vault data over the weekly reset. They now suspect that a "Mythic (Flexible Raiding)" slot forces that slot to be Mythic Nymrissa loot.
This theory hasn't been disproven yet, but there have been numerous vaults over the last two weeks that have a Nymrissa item in that exact slot. If this is true, this means that if you have killed an odd number of Mythic Venomous Abyss bosses and Nymrissa, you will have a guaranteed piece of Nymrissa loot.
Why Does This Matter?
There are some pros and cons to this system.
If you are specifically going for Mythic Nymrissa loot, such as for the Wavecaller's Seastone, then you may wish to kill an odd number of Mythic Venomous Abyss bosses so that you can get a Flexible Raiding slot in the Vault and have a much higher chance of getting the item you're looking for.
There are some downsides to this if you are still looking for Heroic. Many players still want loot off of the last two Heroic bosses, which would drop Myth 1/6 in the Great Vault. Items such as the set bonus on Coiled Altar of Zul'jin's Guillotine Technique and Aman'muso, Warlord's Vengeance or Maze-roa, Warlord's Fury are very powerful, and if you kill Nymrissa Wavecaller and don't need any loot, then you could get a guaranteed useless piece of loot in your Vault instead of a chance of something useful. In this case, you should avoid killing Mythic Nymrissa at all, or kill an even number of bosses in Mythic Venomous Abyss.
Ultimately, with Bonus Rolls, the consequences of just playing the game and not caring about this are a lot lower. Even if you don't want Nymrissa loot, you can just pick up a bonus roll, which allows you to have a chance of getting exactly what you want. This is a very interesting quirk of the Great Vault, and it may be more useful to know once Kith'ix releases in Patch 12.1.5.
