Since Walker was the only one who could sense undead mana, it made it hard to get through the fact that he sensed it everywhere. It either meant that there were a lot of undead here or that the undead moved and spread the mana they radiated all around these ruins.
With the quality of the walls and the area itself, there were a lot of clear passages to move from what the party was guessing too. Therefore, they all understood that the wandering undead was a worse situation. The adventurer group and the guard groups behind them would find themselves struggling if the undead were able to wander in to them first.
Midnight and Onyx seemed like they would suggest heading down different passageways. If they were safe enough to travel through, then they didn't worry that they would struggle. Especially since Midnight and Onyx had taken dragonkin from and Onyx a smaller form already. They wouldn't cause any cave-ins even if they did manage to break the stone walls.
