Deathguard are the “vanguard” of the fleet as They are the ones sent to Terra first (though they get waylaid in the warp and turn to Nurgle) according to John French the main characters of the first book will be Ahriman, sigismund and a few select others.

The main thing is that the traitor fleet is entering into the guns of the Imperial fists fleet, then they smash their way through to Terra, unload as much shit as possible onto the planet and fight to stay alive until they retreat at the end and get harried all the way to the Eye of Terror (or in the Thousand Sons case open up huge warp portals and go directly home).

The Thousand Sons (according to old lore) spend most of the siege hanging back and summoning daemons and only coming out to break open the vaults, do some plundering and then get fought off by the IF and BA librarians.