Source: George W.E. Nickelsburg, "Stories of Biblical and Early Post-Biblical Times," Jewish Writings of the Second Temple Period, ed. Michael E. Stone (Philadelphia: Fortress, 1984) 53.
"As Isaiah is being tortured, Bechir-ra, acting as the mouthpiece of Satan, attempts to get the prophet to recant. With the aid of the Holy Spirit, Isaiah refuses, curses Bechir-ra and the demonic powers he represents, and dies."
Nickelsburg is discussing the Martyrdom of Isaiah, which he dates to the second century B.C.E. Nickelsburg sometimes assumes that literature is talking about Satan when it doesn't specifically do so, for he states that Jubilees refers to birds as agents of Satan, when actually Mastema is the one who sends them. In the case of the Martyrdom of Isaiah, however, Satan is explicitly mentioned, for 2:2-3 say that the evil King Manasseh served Satan and his angels.
I have a colleague who says that the concept of Satan as the embodiment of evil first appeared in New Testament times. Many scholars have argued that, in the Hebrew Bible, Satan was like a prosecuting attorney, who continually pointed out to God the faults of his people, even going so far as to place them in a position to display their sins (as in the case of Job). I heard one professor assert that the word "Satan" was a technical term for prosecutor in a particular ancient Near Eastern country, but the Anchor Bible Dictionary says that the word is unattested in the ancient Near East, so I'm not sure who's right.
In the intertestamental period, my colleague contends, God doesn't have one major adversary in Jewish literature, but rather there are many demons. For my colleague, Satan as the embodiment of evil first emerges in the New Testament, since Jesus knew about the spiritual realm and could inform us about it. (My colleague wrote the paper at a Christian school.)
But apparently Satan is the embodiment of evil in the Martyrdom of Isaiah. Indeed, he tries to trip Isaiah up, but that could be part of his prosecutorial role of showing a person's not worthy of God's favor. But he seems to be more than a prosecutor: he's the one evil King Manasseh is worshipping when he serves idols. He is the source of evil.
I think he may be something like that in the Hebrew Bible as well. I don't want to dismiss him being a prosecutor, but I find it interesting that the Hebrew Bible often uses the word "Satan" to refer to enemies. Here's a quote from Victor Hamilton's article on "Satan" in the Anchor Bible Dictionary:
"The first human called a śātān in the OT is David. Philistines rulers, observing the presence of David and his supporters in their camp as they prepared for war with Israel, complained that David would in fact become their “adversary” (1 Sam 29:4), and thus win the favor of his own king, Saul.
"The second instance involves Shimei, a Benjaminite who had earlier cursed and humiliated David as the king fled Jerusalem (2 Sam 16:5–14). Subsequently a repentant Shimei sought David’s forgiveness (2 Sam 19:19b–21—Eng 19:18b–20). Abishai, a member of David’s court, pushed for Shimei’s execution for blaspheming the king. David, however, opted for leniency, and branded Abishai (and his brothers) as an 'adversary' for even suggesting such a thing (2 Sam 19:23—Eng 19:22). Killing Shimei, while legally permissible, would seriously diminish David’s chance of effectively ingratiating himself with the Saulide Benjaminites. David will decide who, if anybody, shall die for any crime.
"The third instance involves Solomon. He wrote to Hiram, king of Tyre and friend of his late father, stating that David had been unable to build a temple because he was so preoccupied with war in expanding and defending his empire. Now, however, Solomon is free to pursue that project, for his era is one of relative peace, one in which Solomon is without any kind of an “adversary” (1 Kgs 5:18—Eng 5:4). Clearly śātān here designates military enemies, those who threaten the well-being of others.
"Perhaps Solomon, in speaking of the absence of satans on his borders, spoke prematurely. Some years later Yahweh raised up two satans against Solomon, whose relationship with Yahweh was in disarray. The first was Hadad from Edom (1 Kgs 11:14), and the second was Rezon from Syria (1 Kgs 11:23, 25). Here again, the meaning of śātān is military rival who lives outside one’s empire."
"Satan" means "adversary," or "enemy." It's not a proper name, as so many Christians treat it, but it's what someone is. It can even refer to humans. That's why the devil is called ha-Satan--the Satan--rather than just Satan: it's a role he plays, not his proper name. And maybe he is called that, not so much because he's a prosecutor, but because he's the enemy of God and all humanity, especially God's people.