“We march for Macragge.”
– Marius Gage, Chapter Master

The Ultramarines Legion has heard no word of the Heresy yet and musters at Calth to prevent a xenos incursion, by order of Horus.

After opening fire, the Word Bearers sent to aid them reveal their true objective: to cripple the Ultramarines so that they cannot join their Loyalist brothers in the fight against the Warmaster.