Creature for making a tunnel

[Teacher Man]

If you are talking Drow, you are talking spiders... There are spiders that burrow and with a bit of help from the spider queen I am sure that would do the trick.

 

[Samloyal23]

Consider a Pech, they can dig and create support walls.

 

[Greenfield]

Blue dragons have a burrow speed of 20. White and Brass dragons have a burrow speed of 30. No mention of whether they leave a usable tunnel.

Ankheg burrow at 20 feet, but only leave a usable tunnel if they cut that speed by half, so you're back to 10 feet.

The Nightcrawler (2d hit dice undead) has a Burrow speed of 60. No mention of tunnel or no tunnel.

Thoqqua (Earth Elemental) burrow at 20 feet and do leave a usable tunnel.

The Umber Hulk has a burrow speed of 20, and can go through solid roc (5 feet).. It can choose to leave behind a usable tunnel.

 

[Teacher Man]

You know you could also simply have it set up that the drow and their allies pre-dug the tunnels. No sense in over complicating things.

 

[Kevin Blechinger]

Brass Dragon, burrow speed 30 feet.