Tivash’s ability counts the total amount of life gained without considering any life you lost during that turn. For example, if you lost 3 life and gained 4 life earlier in the turn, you may pay 4 life to create a 4/4 Demon.

You can’t pay less life than the amount of life you gained to create a smaller but less harmful Demon.

You need to gain life before the end step begins for the last ability to trigger.