Warhammer: Redemption of the Word Bearers

Okles, a Terran-born veteran of the Word Bearers Legion, loses contact during an expedition due to a warp storm and wakes up to find himself in 014.M31—during the Siege of Terra. The corrupted Primarch, Warmaster Horus, personally leads several legions to besiege Terra, and the situation is critical... — Description from novelcodex.org