Early 17th century. Either from post-classical Latin portention-, portentio a portent, or independently from classical Latin portent-, past participial stem of portendere + -ion.
Definition of portention in US English:
portention
nounpɔːˈtɛnʃn
The action of portending; a portent, an omen.
Origin
Early 17th century. Either from post-classical Latin portention-, portentio a portent, or independently from classical Latin portent-, past participial stem of portendere + -ion.