Each of the triggered abilities of Sarkhan’s Unsealing resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

If a creature card’s power is written as * and an ability defines its power, that ability applies while the creature spell is on the stack. Sarkhan’s unsealing checks its power only as you finish paying costs; it doesn’t matter what the spell’s power is as you begin to cast it or when the triggered ability resolves.

In a Two-Headed Giant game, the last ability of Sarkhan’s Unsealing causes the opposing team to lose 8 life.

If you cast a creature spell that will enter the battlefield with a number of +1/+1 counters, such as Hungering Hydra, those counters aren’t considered when determining whether either ability of Sarkhan’s Unsealing triggers. Similarly, effects that will raise the creature’s power once it has entered the battlefield won’t apply.