Delphan Gruss is an Archmagos of the Explorator fleets within the Adeptus Mechanicus.
Official Canon Edit
Delphan's particular obsession is searching for an artifact called the Omnicopaeia, which contains every STC blueprint in existence. This has gotten him into trouble with local authorities on multiple planets.
TTS Canon Edit
Delphan Gruss and his team went to the Orior subsector, chasing after rumors of a recovered STC fragment, but instead discovered a planet covered in archaeotech for what appeared to be a massive musical stage. Unbeknownst to him, his arrival was part of Cypher's plans. When the Dark Angels arrived, he told them to go away and take it up with the Fabricator-General of Mars if they wanted to complain to the Mechanicus.