Regicide

Travel through the Underground Pass which can be found on the far west side of West Ardougne. You do not need to find another Iban's doll or complete any of the steps from Underground Pass quest. You only need to reach Iban's temple.

Refer to this guide for instructions navigating the Underground Pass.

Enter the well in Iban's Temple.

Exit the cave to the west.

Run through the forest, talk to Idris. Watch him die then continue.

If Idris does not spawn, log out and back in to trigger the scene.

Go west to jump the leaves trap (a dark green patch on the ground).

Proceed southwest of the musician. Between some trees find a trap of sticks on the ground, click the sticks (choose the "pass" option, clicking beyond the sticks has a 100% failure rate).

This may take a few tries (if you spam click the trap, it will let you pass - make sure you're clicking the trap itself, as it has a small hitbox. If you're running low on health, return to the musician to recuperate before trying again.

Continue south towards the lodestone.

Activate the Tirannwn lodestone once you find it.

Run back north-west towards the elf camp.

Jump another leaves trap, run past the bears and cross the log into the elf camp.

Talk to Lord Iorwerth.

Teleport to the Tirannwn lodestone, talk to the elf tracker.

Talk to Lord Iorwerth again which requires at least one free inventory space.

Talk to the elf tracker back at the Tirannwn lodestone.

Follow the tracks just west of the lodestone in front of the dense forest entrance.

Talk to the elf tracker.

Enter the dense forest and kill the guard that appears.

Go north, step-over the tripwire and go through more dense forest.

Stepping over avoids taking damage as opposed to walking through.

Continue north through the dense forest and west, then once again through dense forest to enter Tyras Camp.

Talk to General Hining in the camp.

Take two barrels behind the largest rectangular tent.

The second is required for completing Mourning's End Part I.

Teleport to the Tirannwn lodestone again and head south to reach the shoreline of the swamp.

Use the barrels on the tar to fill them into barrels of tar.

There are only two clickable tiles near the corpse and trees. Mouse over along the shoreline until you find the clickable area.

Take some sulphur from the floor, then grind it into ground sulphur.

Head north-west into the elf camp and talk to Lord Iorwerth.

Read the Big Book of Bangs he gives you and pick up the empty pot north of him if you need one.

Go to a furnace and use the limestone on it to obtain some quicklime.

You need a pot if you wish to grind the quicklime now, otherwise it will be destroyed.

Talk to the Chemist in Rimmington.

The book must be in your inventory, otherwise your Quest Journal will not update.

Use your barrel of tar on the fractionation still outside.

To operate the still:

Click the tar regulator side twice.

Wait until the arrow gauge is within the green on the pressure valve side, then click the pressure valve once. This should keep the pressure indicator in the green without fluctuating.

Now add coal in 2x increments using the middle "Add coal" button to get the tar regulator indicator in the green.

Now that the pressure valve and tar regulator are both in the green, the distillation bar at the bottom of the interface will begin to fill up.

When the distillation bar is full, you may exit out of the interface, You should now have a barrel of naptha in the backpack.

If you plan on completing Mourning's End I a barrel of naphtha is required for that quest, repeat this process for a second barrel.

Make the bomb:

Use ground sulphur on the barrel of naphtha for naphtha mix.

Grind the quicklime with an empty pot in your backpack.

Use the pot of quicklime on the naptha mix.

Use a strip of cloth on the barrel bomb.

Teleport back to the Tirannwn lodestone.

Kill and cook a rabbit just north if you didn't bring a cooked rabbit.

Head to the catapult north of Tyras Camp (west through the dense forest and tripwires if you used the lodestone).

Right-click use the cooked rabbit on the Tyras guard standing next to the catapult.

Use the barrel bomb on the catapult.

Talk to Lord Iorwerth at the Elf Camp.

Attempt to enter Ardougne Castle. Arianwyn will talk to you.

Talk to King Lathas.

Quest complete!