The Culexus Temple is one of those operated by the Officio Assassinorum The temple itself is located on a planet that is hard to locate owing to the high concentration of assassins there.
The Makings of a Culexus Assassin
The Culexus Assassins are the most sinister of all the assassins. They are null-entities in the warp, creating a terrifying aura of unnaturalness and paranormal fear around themselves by merely existing. Psykers within range of a Culexus Assassin are so unnerved by the experience they may simply break and and attempt to flee, making these Assassins perfect witch-slayers. In the confusion caused by this fear, the assassin can move in on its target and eliminate them.
It is possible that Culexus Assassins possess the "Pariah Gene" (making them soulless, accounting for their having no warp-presence), thus also making them perfect candidates for becoming Necron Pariahs. Though if this is truly the case, it becomes interesting to consider how these Pariahs are recruited into the Officio Assassinorum, as carriers of the Pariah gene are incredibly rare, an entire planet may only produce a single candidate in a whole generation. This rarity, coupled with the extremely high rate of student attrition within the organisation may give an indication of how many Culexus Assassins may be in existence at any one time.
In exploring one of many paths of the future an Eldar Farseer saw that if the culexus temple were destroyed then great carnage would be unleashed upon the universe, destroying Eldar civilisation. Creating a paradox that the culexus temple is the greatest threat to the Eldar, but removing it will surely bring their final doom.
Wargear
The Culexus Assasin slays his foe by using a deadly psychic blast directed through an ancient device known as an Animus Speculum. It is unknown how this device functions fully, but it is speculated that the warp energy separated from its targets by the null aura of the Culexus is held in this device and then unleashed. Indeed, the weapon becomes more and more potent when within the vicinity of psykers.
Culexus assassins also make use of psyk-out grenades, which are devastating to psykers, but do nothing to those without psychic potential.