how exactly does the rift device work is supposed to kill it
in one hit or supposed to weaken it.

peace

Equip the Rift Device in your quick use items (e.g. where you placed your wands usually).

Select the Rift Device by clicking it once, then 'target' *whateveryoulike* by clicking on the target.

You will still need to hit *whateveryoulike* once to kill it. I think the Rift Device takes any creature down to 1 HP.

No, it takes the creature around to 10 hp if it is tough enough. Maybe it will kill someone weak like a peasant, never tried out. I use it only at UE, and still have to hit it hard enough to kill.

And there will be consequences if you take it with you out of the place ;)

there are ways to avoid that...

its a ranged harm spell, ignores magic resistance, no save...works on dragons and liches, and probably anything in the game... ;)

Use the Rift Device carefully, though, as it has a casting time and can be interrupted.

It's not just a Harm spell--it's a Spellstrike, Breach, then Harm, all in one. Yowza.

can someone spoil me on what the affects of taking it with you are?

The gods don't take kindly to having cursed artifacts stolen. So...
.
.
.
.
.
.
If you take it with you in your inventory, you get a cute little message about how you were warned, and then you die.