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Tachites are glowing parasites that attach themselves to the brains of humans, essentially linking them to the consciousness of a Somnian.

Tachites are part of a pod connected to a Somnian host body. Such a pod contains thousands of tachites, which a host body released upon Galactica's crew, possessing all but a select few of Galactica's crew.

After a tachite attaches itself to a human host, it controls the host and is able to access the host's knowledge. A physical effect of a tachite's attachment to a host is that the host's eyes glow, and the host ignores non-infected individuals, as so long as these individuals do not arouse suspicion (Prison of Souls, Part 2).