New campaign for Mech Commander Omnitech - Clan Wars by nicosuave.
Description (from the author):
You start as new merc commander and are trying to earn a reputation for yourself in the innersphere before the Clan Invasion. From simply escorts to defense to full on assults your unit is tested to their limits and this is before the Clans come to take the innersphere back.
This campaign was created to try to include some of the glory of the books into the game. It is not perfect, but I felt the Clan Invasion is something that has not been done yet, least when I made it.