The Istrô (or Istaev) theonym is attested through the Istaevone ethnonym, supported by Tacitus’s claim that the Irminones, Ingaevones, and Istaevones are named after brother Gods:

“… they celebrate the god Tuisto (Frk. Tîwiskô), sprung from the earth, and his son Mannus (Frk. Mannô), as the fathers and founders of their race. To Mannus they ascribe three sons, from whose names the people bordering on the ocean are called Ingaevones; those inhabiting the central parts, Herminones (Irminones); the rest, Istaevones.” (Germania, Oxford tr.)

While Tacitus is known for being a somewhat untrustworthy source, the “Ing-” prefix is common in Germanic theonyms, such as the Norse Yngvi and Anglo-Saxon Ing/Ingui. Accounts of the Irminsul also support the existence of a God called Irmin.

Thus, TFA infers the existence of three Gods from these ethnonyms: Irmin, Ing(uo), and Istrô.

Following the logic that these Gods are brothers, TFA infers an analogous relationship of the Irmin-Inguo-Istrô trio to the Roman Jupiter-Neptune-Dis Pater (or Greek Zeus-Poseidon-Plouton) trio through the etymologies of their theonyms.

Hence, TFA presumes that Istrô is keeper of the dead.

The theonym is likely derived from the
PGmc *Istraz, for “holy, powerful.”

The implications of “holy” are relatively broad when applied to a divine being, but could suggest liminal qualities. Following the Ploutonic analogy, TFA takes a chthonic view of this etymology.

TFA Intepretation:
After having been gored by the bull Doldôth (Wild Death), Istrô becomes Swervandôth (Wandering Death). As the first to die, Istrô is also Furistadôd (First of the Dead), and as such, is ruler of the dead at Wîtansal (Wise Hall). He also takes on the title of Mokka (tr. “pig,” interpretatio Mercurius Moccus) in his role as hunter. He is presumed to have psychopompic and liminal qualities.