"I'm confident that those seismic tremors are just the plasma coolant system acting up again. We can safely disregard it."
- —Gideon Hyland, Overseer of Hive Trimalov before its fall
"What Lies Beneath" is an exceptionally large Trygon of Hive Fleet Dagon. Confirmed reports of this specimen were first obtained during the early stages of the Hive World of Castobel's invasion by the Achilus Crusade in the Jericho Reach.
Recovered security recordings from the ruins of Hive Trimalov indicate that this Trygon burst through the hive's lower levels. Within solar hours of its arrival, there was a major explosion within the hive's central plasma core, irrevocably collapsing the city's power supply and slaying all of the xenos attackers.
Without power to maintain internal ventilation, lighting and water systems, let alone defensive weapons, the hive city's doom at the claws of the oncoming swarm was assured.
A low ranking Tech-priest who worked within the plasma core maintenance system was among the few survivors of the explosion. Mech-Wright Tolber endured extensive debriefings with Inquisitorial agents recounting the fine details of the event.
Through these interviews, Inquisitorial agents discovered that the plasma core was in the process of a long overdue retrofit when the attack struck.
As the Trygon burrowed beneath the imperial hive, creating a tunnel for its attack, its passage severed the massive pipes that carried the energy core's coolant fluid. The sacred fluids drained into a deep underground chasm in short order.
Without any coolant, the nuclear fusion reaction soon escalated beyond the capacity of its containment vessels, leading to the explosion.
The presence of Tyranids within the hive city, far from the known battlefronts, was terrifying to the population. However, the damage to the plasma core was directly responsible for the Imperial loss. It seems extremely unlikely that a Tyranid monster could comprehend the complexities of the plasma reactor.
Further, as the creature was unquestionably slain in the resultant blast, there was no reason to expect a recurrence of this tactic. Consequently, the loss was explained as a tragic coincidence.
But during the battles upon Credos, a similar Trygon attack took place deep behind lines that were believed secure.
In this instance, the monstrous Tyranid creature led an assault upon a supply depot, where Imperial assets were stored and refitted. The base commander had violated standard Imperial deployment regulations and established the temporary promethium storage tanks far too close to the facility's field munitorium.
Again, this may have been a tragic coincidence. The foul xenos just happened to strike shortly after the depot had undergone a full resupply. An orbital resupply run had just topped off the promethium tanks. The munitorium had also recently completed a manufacturing cycle devoted to artillery units.
Hundreds of thousands of potent explosive shells were awaiting transfer to an overdue transport shuttle. In combination with the thousands of kilolitres of promethium, the depot was a lethal combination. The consequence of this timing was predictably disastrous.
When the Trygon made its attack, a massive bioelectric pulse ignited the fields of promethium storage tanks. In short order, the intense fire spread to the munitorium. Then, the artillery shells detonated in a cataclysmic fashion. Between the detonations and the walls of flames, the depot was a near total loss.
Hundreds of Imperial tanks and crews, thousands of trained support staff, and a crucial resupply facility were all lost in a few moments. The only positive to be gleaned from the incident was that the Tyranid creature responsible was surely slain in the explosion as well.
After that incident, several Imperial analysts noticed the similarities between the events upon Credos and Castobel. Since that time, several similar instances have been recorded. In all cases, an unusually large Trygon emerged amidst a seemingly secure location.
In an instant, the monstrosity would trigger a disastrous explosion that vaporised substantial Imperial assets as well as the Tyranid responsible. Solar weeks or months later, a similar incident would occur on another world, often systems away.
Between pictographic records and sketches compiled from survivor interviews, additional elements began to crystallise a complete story. The Trygon, which has been dubbed "What Lies Beneath," by the forces of the Achilus Crusade, was somehow capable of targeting Imperial stores of volatile substances.
Once this information was deduced, further investigation linked the creature to additional attacks upon Imperial fortifications. In all instances, the facilities had received substantial chemical stores or had altered the mechanism by which its volatile chemicals were contained.
Further analysis revealed that the creature was consistently accompanied by several Hormagaunt broods during the attacks. None of the confirmed incidents included any other Tyranid strains as part of the assault. Another point of interest was that many of these attacks occurred during the earliest stages of the Tyranid invasion.
These events were identified as anomalous, because they preceded the usual time point at which Trygons would appear as a part of the Tyranids' ecological invasion.
Analysts have suggested that this particular variant of the Trygon is either capable of travelling within a Mycetic Spore or that it has an altered genetic prioritisation so that it appears at a time point earlier than the norm. Additional evidence must be acquired before either mechanism may be confirmed.
Almost as debilitating as its attacks have been upon Imperial supplies, they have also been devastating to Imperial morale. Survivors of the horror's previous attacks have spread word of the nightmares it incurred.
The Achilus Crusade command has attempted to stifle these rumours, but as necessary procedural changes have been made to counter its actions, the stories have circulated ever faster.
Because some recount stories of seeing "What Lies Beneath" slain in a massive fireball, later confirmed sightings have also added legends of immortality to the creature's reputation.
Some commanders have taken extreme reactions to the horror's presence. These include cutting fuel and ammunition reserves to extremely minimal levels, keeping supplies within transports, and repurposing armour for tank repairs as additional plating beneath structures.
Several have even requested their Adeptus Mechanicus allies install seismographic equipment, so that they might be aware of any unusual motions beneath them at all times. These reactions are, of course, frowned upon as their expense limits crusade capabilities even further.
Analysis of compiled imagery and witness reports has met with limited success to identify specific characteristics of this specimen. While it appears that "What Lies Beneath" is larger than other representatives of the Trygon strain, a very limited number of additional identifying traits have been found.
In the majority of instances, the creature bore a distinctive ventral stripe, but this trait was not confirmed in all attacks. This specimen has also never been confirmed as active on a battlefront within a more traditional engagement.
However, it remains possible that field units were unable to distinguish "What Lies Beneath" from a typical Trygon during the course of a battlefield encounter.
- Deathwatch: The Jericho Reach (RPG), pp. 204-206