Sentinel Crystals are a form of Xenotech device found in the upper section of the Torvus Bog in Metroid Prime 2: Echoes. They were developed by the Luminoth during the war against the Ing, and were used to monitor key areas for Ing Horde activity. They are apparently extremely durable, as only the strongest Ing could damage them, but were somewhat less effective in the Ing's home dimension on Dark Aether. When Samus arrived on Aether, some Sentinel Crystals were still active, the last remaining being used by U-Mos to monitor the planet for Ing movements. This is likely how U-Mos was able to inform Samus that the Ing were increasing their forces in the Torvus Bog after she stole the Light of Aether back from the Dark Agon Wastes.
On their away missions to Dark Aether, the Luminoth were able to deploy Sentinel Crystals there as well. However, they soon discovered that the Crystals in place there proved unable to remain active for long periods of time due to constant exposure to the dark planet's atmosphere and its inhabitants' attacks. As a result, the Luminoth turned to the creation of the Dark Sentinel Crystals to function in a dark environment.
Sentinel Crystals and their dark counterparts are similar to Airthorns, in which they were all built by Luminoth in order to inform them on any activity and possible danger from enemy forces on both light and dark planets, except that the latter mechanoid was designed to monitor local airspace.
Xenotech: Luminoth Sentinel Crystal
Used to monitor key sites.
Used by Luminoth soldiers to observe tactical areas during the war. Most are still broadcasting to U-Mos.
During the war, the Luminoth used the Sentinel Crystals to monitor key areas and installations. They are incredibly durable resisting all but the most powerful of Ing attacks. U-Mos still uses the remaining Sentinel Crystals to keep a watch for Ing activity.
|Hazardous||Acid (Beta) • Acid Rain • Afloraltite • Dark Water • Fire • Fuel Gel • Lava • Phazon • Purple liquid|
|Gases||Dark vapor • Industrial-grade pesticide • Meta-viprium gas • Nohadin gas • Noxious gasses • Poison gas • Talvania Gas|
|Destructible|| Bendezium • Brinstone • Cordite • Denzium • Frigidite • Glass • Grobnite • Ice • Maldium • Meltable Metal • Radion • Sandstone • Talloric Alloy • Vegetation • Zebetite |
Kelbium and Zafrite
|Phazon||Red • Crystal • Black Crystal • Mass • Ore • Elite Nutrient Mix • Strain Vertigo • Phazite (Red) • Concentrated|
|Crystals||Ammolite/Witherite Shards • Black Phazon • Crystallized Fuel Gel • Crystals • Dark Sentinel • Lift • Liftvine • Light • Phazon • Red magma • Sentinel|
|Ore||Urthic • Phazon|
|Miscellaneous||Invisible platform • Jovian steel • Liquid Resource • Sand • Vaccine "Metroid" • Waste • Water|