Hi, first time posting so I hope I am not doubling up a question (Dug through a few pages and couldn't find an answer.

The local judges and I were discussing the scenario where a player wants to sacrafice and target the same creature with Fiery Conclusion . Let's assume for the sake of the game it is the only creature on board and you NEED to sacrifice it this turn or lose, and only card in hand is conclusion.

You always use to be able to since you announce targets first, and then pay costs after. However, we noticed the new changes in the origins. It says that there is a second check after announcing the spell before paying costs where it checks if it is legal. Would this stop you from casting the conclusion since you have announced your intent to sac the guy and target him?

601.2e Based on the previous announcements, the game checks to see if the proposed spell can legally be cast based on applicable timing rules (including ones based on the card’s type) and other effects that may allow a spell to be cast or prohibit a spell from being cast. If the proposed spell is illegal, the game returns to the moment before the casting of that spell was proposed (see rule 717, “Handling Illegal Actions”).

My assumption is you can still do it (MTGO allows you to also). That said, locally we decided you couldn't so I figure I should ask! Thanks for all the help.

Yes, you can still cast Fiery Conclusion (and similar cards) targeting the creature you intend to sacrifice; the introduction of 601.2e doesn't change this behaviour at all.

It's worth mentioning that while one of the “previous announcements” mentioned in 601.2e is whether or not you intend to pay (optional) additional costs, such as Kicker, what resources you intend to pay them with is not. At the time 601.2e is checking your spell for legality, the game has no idea what you're planning on sacrificing.