The game revolves around a quartet of Imperial Assassins tasked with killing Lord Severin Drask, a Chaos Space Marine Sorcerer of the Crimson Slaughter, before he completes a dark ritual to tear open a massive Warp rift in the heart of the Segmentum Solar. Gameplay consists of moving the Assassins around a board representing a desecrated Astropathic sanctum, avoiding or eliminating Drask's minions — fanatical cultists and powerful Chaos Space Marines, controlled by the game itself — and searching randomly-generated rooms for both the sanctum's teleportarium and its teleportation control room. Once the Assassins have located both, they must teleport to the otherworldly Temple of Shades for the final confrontation with Lord Drask. Each turn, the players must draw at least one card from the Event Deck, which introduces complications such as patrolling enemies, alarms, and harmful psychic phenomena (along with occasional help in the form of the guards being crazy). The more enemies alerted to the Assassins' presence there are on the board, the more Event cards are drawn each turn; this encourages the Assassins to sneak past whatever minions they can, and dispatch any witnesses with ruthless efficiency.
Two pieces of related fiction were released concurrently with the game. The first, also entitled Assassinorum: Execution Force, is a novelization of the game's events written by Joe Parrino. The second, Assassinorum: The Emperor's Judgement is an audio drama prequel to the game depicting Callidus Assassin Klara Rhasc's encounter with a crazed Eversor Assassin on the planet Tevrat.
As an added bonus, the miniatures used in the game for the Assassins, cultists, and Chaos Space Marines can also be used in games of regular 40K.
See also Space Hulk, another board game set in the Warhammer 40,000 universe.
The game provides examples of:
- Action Bomb: The Eversor Assassin can use his Omegon Tactic, Bio-Meltdown, to wound everything within six squares of him on a 4+. While very useful against tough models like Drask and Chaos Space Marines, it has two major drawbacks: one, it doesn't discriminate between friendly and enemy models; and two, the Eversor has to blow himself up to use it.
- Boom, Headshot!: The Vindicare Assassin's Deadshot ability, which makes his shooting attack longer-ranged, more accurate, and more powerful if he uses it without moving first.
- Conveniently Timed Guard: A common Event Card result spawns a cultist or Chaos Space Marine at one of the map's entrance points, either predetermined by the card or randomly determined by a die roll. The flavor text for the randomly-appearing cultist even calls out this trope, saying that these individuals have been tasked with wandering the facility at random specifically to trip up any infiltrators who might have memorized the guards' normal patrol routes.
- Dressing as the Enemy: Polymorphine, the Callidus Assassin's Omegon Tactic. Very useful for sneaking up on Drask once the Assassins reach the Temple of Shades.
- Eldritch Location: The Temple of Shades is a cursed building set atop a Floating Continent that hangs suspended between realspace and the Warp in a massive bubble of psychic energy.
- Elite Mooks: Chaos Space Marines. In addition to being tougher and more powerful than cultists, they also possess two points of Stamina (the same as an Assassin). This means that, barring very specific circumstances, it is impossible for a lone Assassin to finish off a Chaos Space Marine in a single turn.
- The Guards Must Be Crazy: A few of the cards in the Event Deck invoke this trope; examples include False Alarm, Misdirection, and An Unforgivable Lapse. There's also Ghostly Tide, which takes advantage of the fact that the Chaos guards are literally crazy.
- Heal Thyself: An injured Assassin can spend an action to activate his synskin suit and return to full health, though there's a small chance that this fails to work.
- Hoist by His Own Petard: Several cards in the Event Deck cause "psychic events" that buff the Renegades or hinder the Assassins in various ways. The Culexus Assassin, however, is not only immune to psychic events, but has an ability that lets him reroll his shooting attack on the turn after one triggers, effectively turning Lord Drask's own psychic prowess against him.
- Invincible Minor Minion: Drask's familiar cannot be harmed or stunned by the Assassins in any way, and is effectively a glorified turn counter. It is still a model on the board, however, meaning it can block movement and line-of-sight when the Assassins reach the Temple of Shades. Furthermore, standing in the wrong place can cause the familiar to advance along the ritual track faster, giving the Assassins less time to complete the mission.
- Kill It with Fire: According to the game's manual, Lord Drask is a powerful pyromancer who can wield deadly Warpfire both offensively and defensively. Nothing in the game itself explicitly demonstrates this, however.
- Luck-Based Mission: Some of the Achievements listed in the back of the rulebook rely heavily on the Assassins getting lucky with dice rolls, card draws, and room placement. Of particular note is the Explosive Result Achievement, which hinges on the Eversor rolling a 4+ on a single die, with no chance to try again if he fails.
- Mooks: Chaos cultists. A lone cultist poses relatively little threat to the Assassins and is easily dispatched, but in groups they are a) hard to avoid and b) hard to wipe out completely before they raise the alarm, leading to more alerted Renegades and more cards being drawn from the Event Deck (which is usually a bad thing for the Assassins). Plus, all it takes is a pair of alerted cultists and a couple of unlucky dice rolls to one-shot an Assassin.
- No Peripheral Vision: Models in this game can only trace line of sight to a three-square wide column of spaces directly in front of them, which can be further blocked by walls and objects. They have no line of sight whatsoever to the sides or rear. Technically this applies to both the Assassins and their enemies, but since the Assassins can turn freely as they move it matters less for them (and serves mostly to limit their firing arcs for shooting attacks).
- One-Winged Angel: An issue of Games Workshop's White Dwarf magazine included rules for using a daemon prince as the game's final boss, rather than a Chaos Space Marine sorcerer — the idea being that, in this version, the Assassins have arrived just a little too late to stop Lord Drask's ascension to daemonhood (though his ritual isn't yet complete, so there's still hope).
- Self-Imposed Challenge: Some of the Achievements involve these, such as completing the mission without using any Tactics, with Drask's health doubled from 3 to 6, with cultists appearing in rooms On Alert rather than Stunned, or with only one Assassin sent to the Temple of Shades (or even on the mission as a whole).
- Stealth-Based Game: In order to win, the Assassins must avoid enemy patrols and swiftly eliminate anyone who does see them, lest they be overwhelmed by a rapidly-expanding horde of blood-crazed madmen.
- Super Speed: Whereas most Assassins can only perform a given action once in a single round, the Eversor Assassin has the unique ability to perform the same action twice. This lets him run across the board at a speed the other Assassins can only rarely match, or perform a Spam Attack to take down multiple opponents (or a single tough one). He can also spend a Primaris Tactic to gain a third action for a round.
- Timed Mission: The Assassins have sixteen turns to reach and kill Lord Drask (plus or minus a turn or two, given the vagaries of the Event Deck). If they fail to do so, Drask completes his ritual and brings about The End of the World as We Know It.