Presumably responding to concerns of the focus on Descent: Underground multiplayer support upon the launch of the Kickstarter for the Descent reboot, an update to the description of the game has further details on the single-player portion of the game (thanks ReadyMan). Here's word:

We haven't forgotten the puzzle-solving single-player experience of the original Descent games, either. Descent: Underground's initial release will include a single-player mini-campaign that serves as a prologue to the full campaign.

As we continue to raise funding and can grow the studio, our plan is to deliver a full-blown single-player adventure worthy of the original franchise. Given that our team has such a rich background in epic single-player games, we have very lofty goals for the prequel story that tells about the origins of the PTMC, the Material Defenders, and even the crazy robots.