Devoured Worlds of the Jerioch Reach

The Xenos menace known to the Imperium as "Tyranids" seeks out and devours entire worlds. They are primarily seeking biomass to expand their hive fleets, so they tend to go after worlds rich in life, but they will sometimes devour even marginal worlds.

In addition to biomass, Tyranids will regularly siphon a planet's atmosphere and liquid water. They've also been known to strip worlds down to the bedrock. Unconfirmed reports claim they are even engaging in primitive mining operations. On Ocean World [REDACTED] they cracked open the world's vast promethium reservoirs, letting it mingle with seawater before sucking the planet dry.

Killing an entire world takes time, even for the Tyranid swarm, years, decades even. Possibly longer if the swarm is smaller or the planet rich. A few Devoured Worlds are completely drained. Sometimes the Tyranids will move on before time. In other cases, they have been driven off or destroyed by Imperial forces.

Except in the rarest of cases, the planet is too damaged to continue to sustain life. The ecosphere is too damaged for it to recover - it's an extinction event, and the world will need millions of years to recover. The Imperium will routinely order Exterminatus for such half-devoured planets.

Devoured worlds in the Jericho Reach:

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Castobel: Unlike Vanir, Castobel was fertile enough to make it a juicy target for the Nids. The human defenders used scorched earth tactics and were able to hold out in several fortress-hives. They would not have been able to hold out forever, but the Hive Mind called away the besieging fleet to take part in the Battle of Hethgard.

Eleusis: Escaped mostly intact, when the Adeptus Mechanicus locked away the planet's waters, turning it into a forbidding dust-ball covered with heavily fortified mountaintop shrine-cities. Unknown to Crusade Command, the planet was already infested with Genestealers, which may have been the true reason it was passed over - the Xenos planned to come back later...

Freya: The remote mining world of Freya had little to attract Tyranids in the first place. When the Hive Fleet arrived, it consumed all human life, but otherwise left the system more or less alone.

Scansion Beta: The hive tendril that had consumed Themiskon had moved to this system at the time of the Crusade. Uninhabitable by humans, the system nevertheless contained a lush world, prime cut for the Tyranids. Lord Commander Ebongrave again ordered bio-terminators to be deployed, wiping out all life (nearly depleting the Crusade's stores of such weapons).

Themiskon Point: Formerly an Imperial Frontier World in the Canis Salient, Themiskon was already overrun by Tyranid bioforms by the time the Crusade forces reached it. Lord Commander Ebongrave ordered bio-terminators to be deployed, wiping out all alien life.

Vanir: Vanir was a human-held hive world. Poisonous and almost life-less by the time the Tyranids arrived, it wasn't exactly prime real estate. The Xenos slaughtered the vast human population, devouring them all, but didn't bother with anything outside the hive walls.