The tunnel ends in a small chamber ahead, but not before the floor opens to the chamber below your position. Below, completely unaware of your presence, several goblinoids huddle next to a line of stalagmites.
If the player characters enter from the South tunnel, read:
After a slight descent, the tunnel opens to a chamber running East to West. From the entrance, you can only see half of the long chamber which gives no signs of closing off soon. You can see three stalagmites innocently perched sporadically ahead of you.
The defense post is held by a group of goblinoids and their pet drakes. Three ambush drakes are hiding behind the northeastern stalagmite and attack if any characters trip over the trap, or the party passes them. After the first round, they are joined by five goblins, three hobgoblins, and the drakes’ bugbear master from behind the line of stalagmites. The bugbear shouts out in pain if an ambush drake is killed before the end of round one, revealing his position.
Stalagmites. The line of stalagmites provides half cover to the goblinoids and acts as difficult terrain when moving through a square with a small stalagmite.
Trap. If a goblin from Room 11 comes to warn its allies waiting here, a thin rope has been string between the two southeastern stalagmites. To notice it, a creature must succeed at a DC 20 Wisdom (Perception) check or a DC 10 Intelligence (Investigation) check. Any creature that fails to notice the string and passes between the two stalagmites must make a DC 15 Dexterity saving throw versus falling prone. Any character with a passive Perception of 15 or higher gains advantage on the saving throw.
Treasure. The bugbear is wearing a diadem made of pure silver. It is the Diadem of the Drake King. In addition, the combined treasure amongst the goblinoids equals 5 gp, 13 sp, 17 cp, and three unrefined crystals worth about 25 gp each. One crystal is blood red, another amber, and the final one is sea green.
Hidden Treasure. A passive Perception of 15 or higher, or a DC 10 Intelligence (Investigation) check reveals a hidden hole about 7 feet off the ground near the northern tunnel. This is where the bugbear typically stores his diadem. Within the hold is a small, soft blanket for the diadem, and a slightly luminous, deep purple refined crystal. The luminescent energy within the crystal pulses slightly and floats through it like a lava lamp. After identifying what the crystal is, a creature can make a DC 10 Intelligence (Arcana) check to learn that the crystal is a +3 Silent Image Crystal.
Crystals. A DC 15 Intelligence (Arcana) check reveals the crystals to be spell crystals. Unrefined crystals are unusable until a trained character can use Crystal Crafting Tools on them.
Custom Alternatives. If you do not have access to the ambush drake statistics, use the worg statistics in the basic rules. Hide them around the room where possible. Change the Diadem of the Drake King to work with any animal with an Intelligence of 3 or lower, and a CR of 2 or lower.
Diadem of the Drake King
Head, very rare
This diadem change size to fit the head of its bearer, and offers advantage on all Animal Handling checks involving drakes. When attuned to it, the diadem also allows you to cast Animal Friendship on a drake once per day, which does not require spell components, and you can sense the general direction of any drake within 30 feet of you. If a drake dies within 30 feet of you, you take 1d4 psychic damage.