Finally got around to finish this. What a blast. I clocked at around 20 hours. Still got two more endings and a bunch of hidden quest lines i haven't seen. Throughly recommended. Omnissiah be praised.
Gameplay is tactical, turn-based and plays out on a grid. Before each mission you get to form a party of 1-4 (maybe even more) Techpriests (heroes) and a number of Mechanicus units (peons) that need to be separately deployed. You fight the Necrons - facing five factions and five bosses.
In battle you get separate pools for movement which is determined by stats, and action which is not variable, but you get to fire all of your weapons and skills in the same turn. Skills have cooldowns while weapons don't. Additionally, the more useful actions require you to spend from another pool called Consciousness Points (CP). These points are gathered from a bunch of sources like different obelisks that are spread out through the maps. You'd also get one the first time you disable an enemy. There are other sources of CP such as items that can extract them from enemies, skills, or through the sheer divinity of the Omnissiah (praise be), acquired through reading a holy verdict or canticle.
Combat turns are handed out based on initiative which is set in stone for every unit type, but could be affected through FTL-like random encounter events. Speaking of random encounters, there are a number of other combat variables that are affected by them, such as starting HP, CP and other stuff.
You take quests from your base of operation which is a gigantic ship above the planet where the Necron tombs are. You play as Magos Faustinius who is in charge of the entire Mechanicus operation in this game. You have several advisors who give you quests. Each of these advisors have distinct personalities with opposing ideologies and quests. The game has three endings depending on which advisor (or none) you side with throughout the game. In general, you get to wintess a fair amount of factional politics within the mechanicus, and the writing is very good - very solid. Overall the story of the game while simple, works great because of the quality of the writing. The environments and units are also good looking and the game has an stellar soundtrack which worked perfectly for my tastes.
The quests themselves always have you visit one of the five Necron tombs to explore and achieve an objective. While these objective vary and you get some degree of choice through quest-specific events, the outcome always involves a number of battles. The battle themselves always fall under the following types: kill everything, scan or destroy consoles, or kill the boss.
As you spend time in the tomb in each mission, the Necrons begin to activate more and more. As you move through the rooms of the tomb and explore a gauge raises and once it hits certain thresholds the mission gets harder. This gauge is reset to 0 after each mission, but the number (1-5) gets added to an overall counter-meter that goes from 0 to 100%. Your objective is to finish the game before the meter fills.
Outside of combat you get an incredible amount of customization options for each of your heroes. You get to choose and mix and match between several different Mechanicus disciplines, each with unique items and skills. The skills themselves are pretty great and I did not find a single one that was useless or only usable in specific situations. They all server a purpose. The appearance of the techpriest can also be greatly altered depending on what sort of gear you equip them. I really enjoyed this aspect of the game. Ultimately, the flexibility of the skill/gear system allows for a fair degree of freedom in making your character builds. For example I made a Dominus who had very high resistance to energy attacks, could draw out a lot of CP per turn, and could obliterate more than 5 enemies per turn thanks to its immensely powerful weapons. It goes without saying that being a W40K game, each piece of equipment comes with its own unique lore, and there is NO RANDOMLY GENERATED LOOT - which I loved. Oh and you also get to name your characters XCOM style.
Overall, i consider this one of the best W40K games out there. Throughly recommended.