Dense forest

Dense forests are thick patches of entangled trees which serve as agility obstacles in the forest of Isafdar, in the elven lands of Tirannwn. They can be passed through with 56 Agility, though they require the player to be a certain way through the Regicide quest, in which the player encounters an Elf Tracker working for Lord Iorwerth, who teaches them how to navigate the dense forest. They are the most common kind of obstacle in Isafdar, and cannot be failed by a player, unlike stick traps, leaf traps and tripwire, all of which can be failed, resulting in a substantial loss of life points and, in the case of the tripwire, a poisoning.

Note that you do NOT need 56 Agility to pass back through dense forest once you have already passed through it to prevent you from getting trapped, so you are always able to travel from anywhere in Tirannwn back to the Underground Pass, regardless of your Agility Level.