The timeline for this game is really confusing. I’m waiting to complain about plot holes yet again until I finish my Durge run, which I think will give more answers, but the Nautilloid leaves me puzzled.
Also, how long has this plan been in motion assuming the Emperor is you know who? On the surface, it sounds like the Dead Three have been planning this for ten years tops, but then it sounds like the plan goes back to the original games, then it sounds like the plan predates the Troubles (Faerun, not Irish), then it sounds like the plan is 450 years in the making…
What makes the most sense right now is that the Elder Brain existed in the vicinity of the Sword Coast for at least four centuries. During this time, it turned Balduran and made it part of the collective. Troubles happen. Bhaal leaves pieces of himself everywhere. Troubles end. Bhaal and the rest of the Dead Three reconstituted after the death of Gorion's Ward around 1482 (judging by Adrian's death). The Second Sundering happens the same year, and the recently revived Dead Three, reluctant to cease interfering in mortal affairs, accept their new status as quasi-divine beings in exchange for maintaining their mundane influence. Likely upset with their now limited divinity, the Dead Three IMMEDIATELY start the plan, with Myrkul going to Ketheric directly (giving him time to develop his capabilities), Bane going to Gortash immediately (selling Karlach to Zariel 10 years prior to 1492), and Bhaal already instructing Sarevok vis-a-vis Orin as a child. The Vault is raided with the direct help of the Dead Three, and the Three Avatars likely use the Crown of Karsus on the Elder Brain within a year of the return of Bhaal. This Elder Brain just had a Nautiloid laying around and Gortash and Orin, being the chaotic children that they are, found the concept of using it amusing.
Edit:
Bigger implication is that Jergal knew of this plan centuries, maybe millennia, in advance. Which means Jergal knew of the Second Sundering and the Time of Troubles in advance. Which means Jergal knows the will of Ao before Ao even does anything. Which, all other things being equal, makes Jergal not just omniscient on par with the other gods, but omniscient on a level on par with Ao. Which likely means Jergal knows everything.
