From Halopedia, the Halo wiki
Chi Rho was a human Inner Colony planet in the Ectanus 45 system, governed by the Unified Earth Government. Chi Rho was one of the worlds said to separate the Inner Colonies from the Outer Colonies.
A large blue planet, Chi Rho had an Earth-like surface and its largest continent was home to hundreds of millions, though the colony was not as developed and was comparatively smaller to those in the Sol and Epsilon Eridani systems.
In 2535, during the Human-Covenant War, following the loss of most of the Outer Colonies, refugees had started flooding the planet and all available space was being used for victory gardens and refugee camps. Vice Admiral Jean Mawikizi resolved to make a stand against the Covenant on Chi Rho, digging in deep within the planet's crust to survive a glassing attack, forcing the Covenant to come down and flush them out man-to-man. The planet was eventually lost and during the fighting the UNSC Roman Blue was engaged before eventually crashing.
"Chi Rho (XP)" was a symbol used by the early Christian church to abbreviate the name of Jesus Christ. It is formed from the first two letters of the original Greek title Christos:ΧΡΙΣΤΟΣ.
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