Satanas is a name I have encountered on several occasions, such as when hearing of a lost silent movie of this name by F. W. Murnau.
What is the meaning of the as ending in Satanas?
In Judaism and Christianity, it is also one of the names given to the devil, a supernatural creature that lead a rebelion against God and one of the main instigators of evil in the World.
The -as ending is purely grammatical. As can be seen, it was added in ancient Greek as a common ending of nouns in that language.
Just to clarify:
The -as here is actually the Greek masculine ending -ᾱς. It's a dialectal variation on the -ης (-ēs) you see in names like Socrates, Euripides, Achilles, and so on: probably the most common ending for male names in Ancient Greek.
So when people started translating the Bible and the Torah into Ancient Greek, and came across the word שָׂטָן (satan, "adversary"), they had two choices for how to represent it. One option was to translate it literally, with the Greek word διάβολος (diábolos, "betrayer"). The other option was to take the word satan and just respell it in the Greek alphabet as σαταν.
In the end, translators weren't consistent. Sometimes they used diábolos, sometimes they used satan. But satan doesn't look much like a Greek word, and if anything would be an accusative form, which is just confusing. So since the Hebrew satan is masculine, they stuck the most common Greek masculine ending onto it, creating Satanâs.
When the Bible then came into Latin, the translators had three options: translate the word again into Latin, transcribe the Greek word, or transcribe the Hebrew word. They generally chose the second, so the Latin Vulgate has diabolus and Satanas (whence English "diabolical", "Satanic").
You'll also often see the form Satana with no final -s (as in the Exorcism of Saint Benedict, which starts vade retro Satana, "go back, Satan"). This is what happens when the word assimilates a little bit into Latin: the native Latin equivalent of Greek -ās is -a. It's very close to the original Greek form found in Homer, -ᾱ (-ā), and it's the form we see in older borrowings like poeta "poet". It's more Latin than the Greek -ās, so it's more common for thoroughly assimilated words than for names (the legendary hero Aeneas always keeps his -s).
P.S. One of the easiest ways to distinguish Ancient Greek dialects is how they deal with long alphas in endings. Ionian always turns them into etas, Dorian and Aeolian never do, Attic varies depending on what letter comes before it. So if you see the -es in an ancient name, it most likely came to us via Athens.
P.P.S. A plain -a ending would also mark the vocative, the form used when speaking to someone directly. This would be correct for the context of the Exorcism formula—but the vocative case died and fossilized quite early, and you'll also see Satanas in direct speech and Satana in other positions. It was always the most neglected of the "traditional" Latin cases, and you'll frequently see "errors" like Domine Deus, agnus Dei ("Lord God" in the vocative, "Lamb of God" in the nominative).