I have been using Necromancy for awhile, but I recently noticed the errata that states:



Does this mean that I cannot cast it during my main phase as an instant? I have been utilizing Necromancy to reanimate Woodfall Primus and such so I get two kills and a creature. But does this errata mean that if I cast Necromancy during my main phase, the Primus would not die at the end of turn? Or can I choose to cast it as an instant even if I could cast it as a sorcery? Thanks in advance for the answers.

If the stack isn't empty in your main phase, you can play it still, and it will be sac'ed at EOT. But if you just want to do it in your main phase (I jsut cannot fathom a scenario where this is ever a problem, even with Fatespinner skipping the combat phase, you can still play it in the end of turn step and have it sacrificed in cleanup or in the draw step) it will look to see if the stack was empty and thus whether you could have played a sorcery.

That poses another question, as far as I understand with EOT rules, if I play it as an instant at the beginning of the EOT step, I will not have to sacrifice it until the beginning of the next turn's EOT step because the step has already begun, and I have wait for the next one.

It doesn't say "Sacrifice it at end of turn." It says "it gains substance until end of turn." The wording is important. The first wording allows you to do what you are thinking of, but the second is akin to Giant Growthing a creature during the End phase - it won't be +3/+3 on the next turn.

You can also play Necromancy at the beginning or end of combat to get the exact same functionality (this trick's good for newer cards like Sulfurous Blast).

To answer your first question in a more straightforward manner, you can not choose whether or not it gains substance. If it's sorcery time, your guy sticks around.

For your second question, both you and Anselan are mostly correct. More detail: Anything that says 'at end of turn' will trigger at the beginning of the next EOT step. If it's already EOT, it'll wait until the next one. Anything that says 'until end of turn' will end during the clean-up phase.
i.e. Astral Slide might bring the creature back a whole turn from now, but Giant Growth and substance go away no matter what.

Thanks guys for clearing that up, I appreciate it.