Baal is a Semitic title that means Master or Lord. While it can actually refer to a large number of different deities, Baal, or Bael is used to describe high ranking demons in Christian mythology. Shin Megami Tensei depicts Baal (or Bael), one of the principal kings in Hell, ruling over the East. Some sources cite Baal as a duke, with 66 demons under his command. The word ba'al was also used in the Hebrew Bible to refer to any number of local spirit deities worshipped as cult images, and in this context they were each regarded as false gods.
Curiously, since the Baal Avatar in Shin Megami Tensei III: Nocturne appears so radically different from the previous portrayals of Baal in Shin Megami Tensei, it could be referring to another deity which was often called Baal. Baal Avatar more closely resembles the statues of Hadad, who was a deity commonly called Ba'al. However, the Baal Avatar is just an Avatar of Baal, so it is possible that the demon Baal (Bael) simply created an avatar that looked different. However it may all come down to Baal being merged with Chiaki that altered his appearance.
It should be noted that the god of rain, fertility, agriculture, and thunder and the lord of Heaven we commonly refer to as Baal was actually called Hadad, and the name Baal is used as a substitute in some texts and in common modern usage. This is probably derived from the fact that, in ancient Canaan, only priests were allowed to utter the divine name, in much the same way as in Judaism where only priests were allowed to utter the name of God (YHVH), so common people simply referred to him as Baal.
Baal is the true form of both Bael and Beelzebub, who are in fact demonized halves of Baal. If the hero spared Bael's frog form and took it with him, then upon confronting Beelzebub, the two will ask to re-merge into Baal, and will then join the hero's party and change the Palace Stone into the Palace Ring. When confronting Lucifer, the demon king will notice Baal's restored form and then will then offer an alliance with the hero. Note that Baal cannot be fused in the Cathedral of Shadows.
"Our friendship dies here. We are adversaries, each upholding our own Reason and contending for creation. As fate would have it, we are both beyond shedding tears. We need not hesitate to fight each other. So, let us now decide who is truly superior. Challenge me with all your might!"
—Baal Avatar, Shin Megami Tensei III Nocturne
Baal, known as Baal Avatar is a Demonic Sponsor of the Deity Race and the 'God' of the Reason of Yosuga in Shin Megami Tensei III: Nocturne. After inheriting the Mantra Army from Gozu-Tennoh, Chiaki Tachibana takes advantage of the Manikin efforts in accumulating Magatsuhi to summon their Demonic Sponsor and establish their Reason, Chiaki leads the Mantra Troops to launch an attack to Asakusa, homeland of the Manikins. Entering the holy-land of the Manikins, Mifunashiro, Chiaki mercilessly massacres the Manikins and claims their collected Magatsuhi as her own. Her justification of her actions is that, since the Manikins' sole purpose was to serve the demons, they have absolutely no rights in establishing their Reason.
Summoning Baal Avatar into existence, she leaves, climbing the Tower of Kagutsuchi. In the Tower of Kagutsuchi, Baal Avatar is the final boss of the three Reasons, and, unlike other bosses, the Demi-Fiend must face Chiaki, as Baal Avatar in the events of the Tower of Kagutsuchi regardless of his reasons; should the Demi-Fiend support the Reason of Yosuga, Baal Avatar will attack the Demi-Fiend see if the Demi-Fiend is fit to live in the World of Yosuga. Should the Demi-Fiend be against the Reason of Yosuga, Baal Avatar will attack the Demi-Fiend for betraying her. During the battle, Baal Avatar also summons two variation of Ose and Flauros alongside with her, aptly named Ose Hallel and Flauros Hallel, respectively.
In either event, Baal Avatar, along with Chiaki is defeated and Baal is shattered to pieces, killing Chiaki as well. However, her last words towards the Demi-Fiend differs based on his reason; should the Demi-Fiend support the reason of Yosuga, Chiaki is pleased to see the Demi-Fiend's capabilities, and acknowledges the Demi-Fiend as a better ruler than her. Should the Demi-Fiend stand against the Reason of Yosuga, Chiaki laments why the Demi-Fiend would not understand her Reason, and dies. Defeating Baal Avatar grants the key item Heavenstone.
Baal is faced in the Challenge Quest, Mysterious Story of Tennozu. He is pretending to be Gozutennou, whose summoning is sought by the people of Toyosu. After being defeated as the Red Knight he devours his followers, stating that they should become part of his flesh since it was thanks to his offerings that they survived up till now. He finds Flynn to be a suitable opponent and a perfect offering for a king of gods like himself, stating that he shall devour Flynn. Shocked at his defeat and realizing he underestimated his foe, Baal flees for fear of his body decaying in order to return to his other body.
After the Red Knight loses in the following quest Rebirth of an Overlord, Baal's voice comes form the Knights head and tells Flynn of what became of Ahazuya Mikado and that he felt sympathy for the pitiful and lonely ruler, which is why he revived Ahazuya as his host and aided him in his desire for revenge against God. Since Ahazuya can not be free even after death, Baal shall take his true form and destroy Flynn, whose existence is an "extreme danger", and transforms into Beelzebub.
Along with Mephisto, Belial, and Lucifer, Baal is one of the four Gaia Meisters using the monsters spawned through Gaia to ransack the world. He is the penultimate boss, fought just before Lucifer. When felled, he admits that he does not create monsters merely for power, but because he appreciates them in their own right. After Lucifer is defeated, he can be found in an amnesiac state at Mt. Palo, where he can be recruited.
"The chief Semitic god. He was worshiped as a fertility god, and his name means "Lord." He is the god of Canaan and brother and consort to the goddess Anat. Many demons, such as Bael, Beelzebub, Belphegor, and Berith are believed to be derivations of Baal. Long ago, he was worshiped in the same temples as Yahweh."
"The chief Semitic god. He was worshipped as a fertility god, and his name means "Lord." He is the god of Canaan and brother and consort to the goddess Anat. Many demons, such as Bael, Beelzebub, Belphegor, and Berith are believed to be derivations of Baal. Long ago, he was worshipped in the same temples as Yahweh."
—Devil Survivor 2 Comendium
In Devil Survivor 2, Baal is a Unique demon that doesn't require a particular fusion combination to create. Instead, the player has to achieve a Fate level of 5 with Yamato Hotsuin before it is available. Once this has been achieved, it is unlocked for fusion on all subsequent runs as long as the player imports his/her save.
Baal is notable in that its innate skills make it impossible for the player to cover its only weakness without any assistance. However, if a Release Passive Add-On is used to erase these skills, Baal has just enough passive skill slots to make itself invulnerable to everything save for Almighty attacks.
As a nod to Baal's ties to Yamato's Fate ranks, Baal is another demon summoned by Yamato in the anime. Yamato uses it to pin Megrez down until all three Megrez bodies can be destroyed together. What is known is that Baal possesses enough firepower to decimate Megrez's shell in one strike to expose its core.
In the Sega CD version of Shin Megami Tensei, a new Deity Race enemy called バール appears, with the appearance of a Medusa-like figure with a crown and wings. This same version of "Baal" originally appeared in Megami Tensei II and it's remake Kyūyaku Megami Tensei as Deity "バアル"; noteworthy for being obtained only via a special union between Tyrant Bael in toad form and Tyrant Beelzebub upon defeat. The same sprite in Kyūyaku Megami Tensei was used for both Deity バアル and Vile バアル of the first game, however the original sprite for Vile バアル did not share this resemblance. (Kyūyaku opted to use the Megami Tensei II style sprites for both titles featured in the remake.)