Note: after the 2008 summer update, a new patch was introduced making this quest harder to make. Few people have finished it since then. It is believed that you have to walk on the 5 squares (Order is unknown) as well as a hidden the 6th square that is supposedly a random spawn like a Boss monster to keep the rewards rare. As of yet, nothing about getting in can be confirmed. Despite this, the team from Amera has let out a hint, that getting in has something to do with the gravestones. A rare hidden 6th switch could also explain the small patch CIP released. The following method does not show any modifications made from the patch
Note 2: after the 2009 summer update, the quest was changed in such a way it is now possible to get into the infested area if you finished the 6666 Demons task of Killing in the Name of... Quest.
First, you need to go to the Plains of Havoc and search for "the infested area" (here). It is surrounded by a lot of Dead Trees, east of Dark Cathedral and north of the vampire's house. There you will find a 1 sqm corridor that leads into the center. (There is no Gate Of Expertise but you do need to be level 120 or above to be eligible for entry)
Surrounding the infested area there are 5 squares that cause the Demon Oak to yell.
Upon entering the area you will receive this event message:
It seems that you have discovered an 'Ultimate Challenge'
Once you have acquired the Hallowed Axe return to the Demon Oak and start to attack its 4 most vulnerable points:
1: The left spiked ball
2: The bird
3: The face of the tree
4: The right spiked ball
Attacking these four points will anger the Demon Oak and cause him to summon monsters. Every time you attack one of the points, you will lose 300 hitpoints (Earth Damage). Once attacking the four points you will have to keep attacking that same area until it is "Dead" or "puffs smoke". Depending on which order you choose to do the four points the demon oak summons different monsters.
Here's the pattern:
Left spiked ball:
5th hit = 3x Braindeaths + 1x Bonebeast
10th hit = 2x Betrayed Wraiths
5th hit = 3x Liches
10th hit = 1x Dark Torturer + 1x Blightwalker
Right spiked ball:
5th hit = 3x Banshees
10th hit = 1x Grim Reaper
The face of the tree:
5th hit = 2x Giant Spiders + 1x Lich
10th hit = 1x Undead Dragon + 1x Hand of Cursed Fate
For hits 1-4 and 6-9 it's random if the demon oak summons nothing or 4x bonebeasts.
The 10th hit on the last of the four points is always 1x demon instead of what should've spawned. I.e. if you do the face last you get demon instead of undead dragon + hand of cursed fate, and if you do the left spiked ball last you get demon instead of 2x betrayed wraith and so on.
Once the tree is 100% dead, you may leave again. Upon exiting the area you will receive the following message:
Tell Oldrak about your great victory against the demon oak.
Thus, speak to Oldrak again to tell him you have defeated the Demon Oak. Once this is done, go to a grave stone which is located south of the infected area with the name of "Yesim Adeit" (Which can be read as "Yes I made it").
"Use" the grave stone to enter the rewards room and choose your reward.