Im going to buy ToB tommorow but I just wanted to know what it gives you. I know about the wild mage but does it extend the game after you kill Irenicus and what other things does it do.

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Know thy enemy and know thyself and you will never be defeated in a hundred battles.

To Quote their sales pitch:

"Uncover the mysteries of Watcher's Keep, a deadly new dungeon, where you will encounter an ancient evil of awesome power. Cast powerful new spells like Wish and Bigby's Hand Spells as your character climbs as high as 40th level with an experience point cap of 8 million! Gain amazing high-level abilities; new combat feats for fighters, deadly new traps for thieves, special quest spells for clerics and 9th level spells for mages. Play a new character kit, the Wild Mage. Fight and Summon awe-inspiring D&D creatures during your search for artifacts, relics and other legendary magic items!"

"I know about the wild mage but does it extend the game after you kill Irenicus"

The expansion is broken into two parts:

The First part (the Tower) can be played at anytime (even during the Shadows of Amn Campaign [BG2])

The second part, yes, it picks up right were you left off. In fact, you can play one straight into the other. Or you can just start TOB directly.

"what other things does it do"

Any game review site will tell you:

Complete the compelling saga and fulfill your destiny as a Child of Bhaal.

Venture into a deadly new dungeon – Watcher’s Keep and come face to face with an ancient evil of awesome power.

Cast powerful new spells like Wish and Bigby’s Hand Spells as your character climbs as high as 40th level, with an experience point cap of 8 million!

Gain amazing high-level abilities;
new combat feats for fighters, deadly new traps for thieves, special quest spells for clerics and 9th level spells for mages.

Play a new character kit, the Wild Mage.

Fight and summon awe-inspiring D&D creatures during your search for artifacts, relics and other legendary magic items.