In the Marathon trilogy of games, this is the name of the eleventh clan of S'pht, who left Lh'owon over a thousand years ago.
According to the mythology/history found on Lh'owon during Marathon 2: Durandal, the S'pht'Kr abandoned their planet for its moon K'lia when the other ten S'pht clans began fighting each other. They and the moon disappeared from orbit sometime later, and it was understood among the elders of the remaining S'pht that they would return when the S'pht clans were reunited. This was accomplished too late when the Pfhor arrived to enslave all the remaining S'pht, but the S'pht'Kr were successfully summoned back when a dormant AI dubbed Thoth was reactivated on Lh'owon.
The S'pht'Kr developed their technology independently of the other S'pht and the Pfhor for a thousand years, and it shows. When they finally arrive to fight along side you, they appear as floating blue spheres with a curved prong beneath them, firing extremely painful disks of green energy at any nearby threat.