Forest of Doom is a quest available after the quests of Serpent's Den have been completed. It can be done once the player has reached Lvl. 24 and unlocked Forest of Doom dungeon, although it may appear in the quest list before the dungeon is unlocked.
To complete the quest, Forest of Doom needs to be passed, since the quest is to defeat Morrigan, the end boss.
Click on [Campaign] -> Select the [Forest of Doom] -> Gain passage through the dungeon You've arrived in the deadly core of the Murken Forest. Continue to discover the source of all evil.