MegaTraveller 2: Quest for the Ancients

Three hundred thousand years ago, the ancients ruled the galaxy. It's now ~4000 AD and they're long gone. Unfortunately, one of their pyramids has started spewing slime that threatens to engulf the entire planet of Rhylanor. The player's party of adventurers must scour four sub-sectors of the Spinward Marches looking for clues to solve the crisis before Rhylanor is destroyed.

MegaTraveller 2: Quest for the Ancients is a faithful adaptation of the Traveller board role-playing game with over 100 fully explorable planets (most with multiple cities and each with its own sub-quests) and a non-linear storyline.