When I activate North Star do I have to use it’s effect immediately, or do I have until EOT to use it?
Also, do I have to use it on the next spell I cast or can I use the effect on any spell I choose?
The wording leads me to believe it’s a blanket effect that lasts until I use it or EOT, and that I can choose which spell in a given turn to apply it to.
When I activate North Star do I have to use it’s effect immediately, or do I have until EOT to use it?
The latter. When North Star's ability resolves, it sets up an effect that lets you "spend mana as though it were mana of any type to pay [the] mana cost" of one spell you cast "this turn". The effect doesn't work immediately (as with a one-shot effect), but lasts "this turn" (C.R. 611.2a, 611.1; compare with C.R. 610.1, one-shot effects).
Also, do I have to use it on the next spell I cast or can I use the effect on any spell I choose?
Any "one spell" you cast "this turn", not necessarily the next one. Compare North Star with Quicken or Saheeli, the Gifted.
The wording leads me to believe it’s a blanket effect that lasts until I use it or EOT, and that I can choose which spell in a given turn to apply it to.
Also, do I have to use it on the next spell I cast or can I use the effect on any spell I choose?
The wording leads me to believe it’s a blanket effect that lasts until I use it or EOT, and that I can choose which spell in a given turn to apply it to.
The latter. When North Star's ability resolves, it sets up an effect that lets you "spend mana as though it were mana of any type to pay [the] mana cost" of one spell you cast "this turn". The effect doesn't work immediately (as with a one-shot effect), but lasts "this turn" (C.R. 611.2a, 611.1; compare with C.R. 610.1, one-shot effects).
Any "one spell" you cast "this turn", not necessarily the next one. Compare North Star with Quicken or Saheeli, the Gifted.
This is how it effectively works.