Nightbringer: An Ultramarines Novel (Warhammer 40,000)

Nightbringer: An Ultramarines Novel (Warhammer 40,000) - Graham McNeill Uriel Ventris, newly promoted captain of the Ultramarines chapter, is sent to investigate a planet's failure to follow the Imperial way.
He finds factions trying to overthrow the ruling power and alliances with the Eldar.
As he soon discovers, there is more at stake than meets the eye.

There's a lot going on, with the different forces all angling for power, and the local Arbites trying to keep things under control on the ground.

When the final option is the destruction of the planet, Uriel takes his command up against the Necrons to try and save it.