"All men must die."
valar means all men
morghūlis means must die
Note that the aorist tense is not always translated with must.
From MadLatinist who is one of the course contributors:
This [the aorist tense] is also the verb form in the infamous Valar Morghūlis/Dohaeris, conventionally glossed "All men must die/serve." Because the aorist implies something is always true, it can sometimes be translated with "must," especially when used with a collective noun.
valar is in the collective grammatical number (in the nominative case), see: https://wiki.dothraki.org/High_Valyrian_Number