I imagine a strange colossal tentacle horror appearing on a dying planet that's been thrown into a permanent (and worsening by the day) winter. It attaches to one of their people and uses him as a means of communication. It offers safe transport to a new land, a way off their dying planet, a way to save their people. They of course are skeptical when they learn the portal is in its gaping maw, but the majority decide that even if it is lying to them, it's better than slowly freezing to death.They slowly march through the pulsating toothy energy field. What do they find on the other side? That's up to your own imagination, kiddos!(Probably also depends on if you're an optimist or a pessimist, lol.)