Round Two

After three or four hours of relative quiet, with darkness swallowing all view of the valley below, the sound of battle is heard from deep in the tunnels below the fortress. Word spreads quickly, and a runner from the svirfneblin ranks comes to the party's base of operations (I assume this would be the room at the base of the ladder into the fortress, or the fortified room above it). The runner reports that an assault has begun in the outer, natural tunnels.
Concerned that their reinforcements haven't arrived yet, the runner is asking that you join them in the tunnels to defeat the intruders. Three of the svirfneblin remain on guard in the fortress to keep a lookout on the valley below. As you race to join the battle, further word comes that this is more than just a unit of kobolds. There is something much larger threatening everyone as well. The svirfneblin have been reduced to ten warriors in addition to the three above, and their leader.
The kobolds have come with a band of 15 kobolds, 4 bugbears, and an oni. The svirfneblin have been split into two groups, with seven of them not accounted for. When the party arrives, there are 8 kobolds and two bugbears before them. As they fight, the oni arrives. It uses its spells to blast everyone from a distance.
The battle can move throughout the tunnels, with both sides feinting and trying to outflank each other.  The other group of svirfneblin can resolve their battle and regroup with the main gang at any point you choose.