The Ascension mod has six or seven components, all of them affect ToB expansion, so I think that you must have a game at least before finishing SoA.
The components affect each of the ToB major battles, the battle with Demogorgon, and the final Ascension fight.
Illasera is now a high level Archer, armed with heavily enchanted Arrows of Dispelling. She also has the ability to go Ethereal at will (improved invisible for a couple of rounds). True Sight will not dispel the Ethereal status, but Detect Invisibility will.
Yaga-Shura has great physical and elemental resistances, and some new abilities, like the ability to cast a Dragon Breath similar spell or use a Wing Buffet similar ability in combat. He's vulnerable to Cold and Poison, though.
Abazigal's lair is improved, adding a new dragon to aid him, and several drakelings(his entire family). Also, Ascension Abazigal has some dangerous spells like Bigby's Clenched fist, Maze and Imprisonment, some of them are unlimited(or quite numerous).
Sendai is a powerful C/M now, and her drow support seems somewhat tougher.
Balthazar has new abilities, increased speed, etc.
Plus, Balthazar may be convinced to join you in the last Ascension fight. He's a powerful ally, but persuading him to join is not easy. Your character must fulfill certain Alignment, Wisdom, Charisma and Reputation check in order to do so.
After completing each of the challenges in the Pocket Plane, new Baalspawn abilities are rewarded to your PC.
Tougher Demogorgon is power to behold. He has infinite Stoneskins, starts with Timestop, summons a lot of demons, continues with Storm of Vengeance, and afterwards fights in melee. His Gaze attacks are nothing to sneer at. He also has unlimited Implosion spells, which he likes to use a lot. If more than 2 minor demons are killed, he summons more.
Chapter 10 starts with battle with your old foe, Irenicus. Mel also summons 2 Fallen Solars to aid him(3 if your group is larger), and Imoen as well(changed into the Slayer). Irenicus summons Bodhi.
Bodhi may be persuaded to join you, again, with some alignment, stat and reputation checks.
The Fallen Solars are powerful F/M, with Druidic and Clerical spells too. Their weapons are Vorpal, and have a chance to dispel magic on hit. Their arrows are also enchanted, and always dispel on hit.
Bodhi is very quick, and may turn into Gaseous form when at low health, thus healing herself and re-entering combat.
Irenicus has an undroppable Robe of Vecna(casting speed reduced by 4), and in the very start summons a Simulacrum and Projected Image to assist him. Also, he has a Wand of Spell Striking, and his melee attack deals magical damage as well.
Afterwads, there are powerful demons guarding each of the pools. Mel will not appear in between pool fights. Also, the pools refresh the entire party + allies, and at lower difficulty, there is also a rest option included. Plus, the PC gains new powers from each of the pools, some of which are permanent, while others are special abilities with relatively short refreshing rate.
The final battle is Mel&the five. The Five, are, of course, Illasera, Abazigal, Sendai and Yaga-Shura. The fifth is either Balthazar, or(if you've joined him) Gromnir. Also, Mel summons Sarevok to fight at her side. There is a way to avoid that last thing, however.
Mel leaves you to battle the Five. Abazigal will not turn into a Dragon, but he's a powerful F/M nonetheless, and uses Breach and Lower Resistance. Sendai has a lot of offensive spells, and healing spells too, plus a scroll of Greater Restoration. When the first of the Five is dead, Mel will appear. She uses Divine Mantle(combination of Spell Trap + Absolute Immunity), has unlimited Pierce Shield and Breach spells, and(at higher difficulty) summons powerful Demons: at first, 2 Fallen Solars, afterwards, demons familiar from the Pool Fights.
Mel cannot be killed before all of the Five + Sarevok are dead.