Details of Random Encounter Determination :
A. Monster Level Table Determination throw: 1d6=2, so it's MLT=1.
B. (Dungeon specific) Monster Level Table 1 throw: 1d12=9, so it's Seers. This could get interesting...
C.1. Number Occurring. Seers are a "player-type" monster; this is dungeon level 1; there are 12 players (excluding one NPC). 12 players will attract four times "the basic number" of monsters, which--in the case of "player-type" monsters--I deem to be 1. 4 x 1=4, thus indicating 4 Seers. However, I prefer a bit of randomness so 4 becomes 2-6 (by my scheme 1 gets 1, 2 gets 1-3, 3 gets 2-4, 4 gets 2-6, and so on). However, DD says that player-types are accompanied by 2-12 guards, and this fits well with the "master and minion" organization one might expect to see with M-Us.
C.2. 2d3=2, so just the two Seers (!); 2d6=8 so they are accompanied by 8 guards; which are (1=goblins, 2=hobgoblins, 3=skeletons, 4=mercenaries, 5=cavemen, 6=animals) 1d6=3, so it's sk-sk-skeletons! M-Us and skeletons; that... works. ish.
D. From whence do they come? Ignoring the possibility of bursting thru walls, there are four plausible directions this wandering inconvenience could emerge from; 1. the unexplored, dark passage (behind the players), 2. the main passage from the bunks room (mid-rear of the marching order), 3. From the servery door (between the two player "fronts"), 4=directly to the front of Zax. The encounter make up and the map make options 1, 3, and 4 seem most plausible, in that order. The ref dices; 1d6=1, so they cometh (somewhat appropriately?) from the dark passage.
E. Surprise. Are the players (at the rear) surprised? 1d6=2, so no (that was luckeeeeee). Are the monsters surprised? 1d6=1, so no.
F. Encounter distance can be 30--80ft underground (OD&D says 20-80ft, but DD5 says 30-80ft). The ref dices 1d6=1, so the encounter begins at a mere 30ft distance! The ref presumes this is due to the curve of the passage preventing better visibility out further...
This is shaping up to be an interesting one. Let see, shall we?