On a different note, just how did you visit Omeluum just off the beach? The closest underdark access would be the Blighted Village, but that would require at least one major encounter (the initial goblin ambush), plus dealing with the ettercap and phase spiders.
Let me copy over my reply to that question from reddit and then expand on it.
"I did the Druid Grove fight and ran inside to buy a potion of invisibility from Ethel. You could skip this part if you wanted. It is possible to sneak past the attacking goblins at the Grove if you wanted to by sneaking along the path furthest from the gate. The cut screen will play but the Grove and goblins fight without you.
Go to the Blighted Village and fight or talk your way in. I was a Drow so just talked my way past.
Interact with the well in the middle of the village to go down to where the spider den is.
At this point you'll need either that potion of invisibility or you'll need to sneak past the spiders and ettercap to get to the hole that leads down to the UD.
The one thing you absolutely have to have is Feather Fall. What I did was had Gale cast the spell on me, then drank the potion and immediately went into turn mode. On each of the MC's turns I used dash to go as far as possible each round(dash does not break invisible). It took me 6 rounds to get to and jump down the hole(still in turn mode). The rest of the party went back up the well and waited for me by the warp stone(the vision cone of the mino ends just behind where you start in the UD. If you bring your whole party at least 1 will likely be in the cone and you'll get jumped).
Once I got into the UD since I was still in turn mode and had Feather Fall still running I just jumped from where you enter down to the hostile mushrooms in front of the Myconid Village. There is enough room there to safely land without triggering everything. Then just leave turn mode, ranged clear the mushrooms and walz right up to the Myconids. I found if you don't leave turn mode before clearing the mushrooms, they can bug out and their aoe will linger in place and not dissipate.
Edit: I ended up doing it as a Drow ranger cause I wanted to try that class. Since time passes regardless of if you camp rest or not, a Drow wizard could have gotten down there a bit faster by completely skipping the Grove fight and buying the potion. You can sneak your way from the beach to the Blighted Village with out engaging anyone."
To expand on what I said in that final Edit, a solo Drow wizard with Feather Fall could carefully but rapidly go from starting on the beach directly to the UD and speaking with Omeluum and never enter combat or much time except clearing the shrooms from in front of the Myconid village. Speed runners probably have some tricks too, I haven't looked.
Getting past the Goblins at the Blighted Village, the ettercap and spiders are all possible with sneak. I did it on my first attempt where I was going to try this playing as a Gith Ranger. I got to the point in the Matriarchs lair where I could jump in and realized I'd die without Feather Fall. I decided to try Drow out instead so I could quickly talk past the Goblins though and have their dark vision down the well and in the UD. In my first run to bypass the Goblins I came up the vines from down near Scratch and was in sneak mode from there where the sleeping bugbear is to the well and down.
