I take the following sentence from Fabules Faciles:
...hōc enim ūnō modō tantum scelus expiārī potuit
as "...only in this way could he atone for such a great crime." literally, "indeed in this one way for such an extent of crime to atone was he able". So, expiari seems to be an infinitive ("to atone"). However, I thought that the infinitive of expior was expiare, not expiari. Is it a subjunctive infinitive or something? This form, expiari, is not even listed in the grammar I use (https://www.online-latin-dictionary.com/).