Whenever possible, use the full, official name of the game
The full name of the game as presented on their website is Warhammer 40,000: Inquisitor - Martyr. Therefore, we would ideally tag the game as warhammer-40000-inquisitor-martyr.
However, you've noticed that this hasn't been consistently followed, largely because of the limit on the number of characters on a tag name. The limit used to be 25 characters, which caused problems for games like Warhammer 40,000: Dawn of War - Soulstorm. The character limit is now 35 characters, and these games should have been retagged.
That said, warhammer-40000-dawn-of-war-soulstorm is 37 characters, which is over our limit. Given that there is the common abbreviation of
40k, that is the obvious choice. For consistency of games of the franchise, I think that all of these games should be prefixed with
This would mean that the name for this game would be warhammer-40k-inquisitor-martyr, with variants as tag synonyms.