The seemingly brutal, savage Tyranids were actually commanded by a highly sophisticated intelligence. This was still a secret to most people in the Imperium, but Guilliman had known it for a long time.
Guilliman retrieved more information on the Tyranids, hoping to determine whether their supreme intelligence intended to shift the focus of its attacks to the Imperium. Then he came across a strange report.
An Imperial World in Segmentum Tempestus was suffering an Ork invasion. The situation was already critical when a Tyranid Tendril Fleet arrived.
At the time, the Planetary Governor holding out on the planet had all but despaired. He was ready to shoot himself.
Then something strange happened. Instead of launching their usual all-out assault on the planet and the Orks, the Tyranids held back. No rain of spores descended from the sky.
After a probing attack... the Tyranids retreated... retreated! In the Imperium's records, the Tyranids had never retreated! Even when they could not win, they would fight on the battlefield to the very last creature whenever possible. Their frenzy could shake even the most battle-hardened veteran.
The Tyranids retreated, but the Orks were having none of it. They immediately assembled a fleet and pursued them in their Gargant spacecraft, leaving only a small number of Orks behind to continue attacking the planet's hive cities.
That was what allowed the Planetary Governor to hold out until reinforcements arrived. The incident was so bizarre that the report had been sent to Guilliman's desk.
The Tyranids' behavior was highly abnormal. As Guilliman went through more reports, he found further instances of the same anomaly. The Tyranids deliberately avoided Ork Waaagh!s and only initiated attacks against weak Ork forces. Whenever they reached a stalemate, they withdrew of their own accord.
Their cautious behavior almost suggested that they were