Nethroi's triggered ability can return any combination of creatures whose powers total 10 or less. For example, it could target ten 1/1 creature cards, two 5/5 creature cards, and so on. Regardless of what other creature cards you choose as targets, you can also choose any number of creature cards with power 0 as targets.

Use the power of the creature cards as they exist in your graveyard to determine whether they can be the targets of Nethroi's triggered ability. For example, a 0/0 creature card that will enter the battlefield as a copy of a 15/15 creature can be returned, as can five 0/0 creature cards that will each enter the battlefield with three +1/+1 counters on them.

If the power of the target creature cards change while in your graveyard, most likely because one has a characteristic-defining ability that defines a * in its power, the total power of the creatures may become greater than 10. If this happens, the entire selection of targets is illegal and nothing is returned to the battlefield.