Regicide/Quick guide

Starting out

 * Talk to King Lathas in Ardougne Castle (wait a few minutes for King's Message to be delivered to you).
 * Travel through the Underground Pass which can be found on the far west side of West Ardougne. Refer to this guide if you get stuck.
 * Enter the well in Iban's Temple.
 * Exit the cave to the west.

Tirannwn

 * Run through the forest, talk to Idris.
 * Run south, jump the leaves trap, pass the sticks.
 * Follow the path north west until you reach a camp.
 * Talk to Lord Iorwerth.
 * Talk to the elf tracker. (In an abandoned camp southeast of Lord Iorwerth)
 * Talk to Lord Iorwerth again.
 * Talk to the elf tracker again.
 * Click the tracks a few steps west of the tracker near the blue mushrooms.
 * Talk to the elf tracker.
 * Climb through the dense forest, kill the guard.
 * Go north, pass tripwire and go through more dense forest.
 * Run west then south through more dense forest, talk to General Hining in the camp.
 * Take 2 barrels (you will only need 2 if you plan on completing Mourning's End Part I).

Sabotage

 * Use your barrels on the tar just south of the Elf Tracker.
 * Get 2 pieces of sulphur from right next to where you got the tar (you only need 2 if you plan on do Mourning's End Part I).

The catapult
 * Talk to Lord Iorwerth. Read the book and keep it in your inventory.
 * Use the sulphur with your pestle and mortar
 * Use limestone on any furnace (make sure gloves are equipped or you will take damage.)
 * Get roughly 18-20 coal from your bank.
 * Talk to the chemist in Rimmington. Make sure you have the book in your inventory. (If you don't, the Quest Journal will not update!)
 * Use your barrel of tar on the fractionation still outside (complete this process twice, if you plan on completing Mourning's End I because a barrel of naphta is required for that quest).
 * Operate the still:
 * Turn the valve on the right all the way to the third tick (all the way to the right).
 * Wait for the pressure gauge to land in the green zone.
 * Turn the valve on the left to the second tick (in the middle) to halt the pressure in the green zone. If it landed in the red zone, you could turn the valve right to relieve some pressure, then back to the middle.
 * The valves never need to be touched again, and the pressure gauge can be ignored.
 * Add 2 coal, and then 1 additional coal whenever the heat gauge falls too low (below, or counterclockwise from, the green zone). Don't spam click the "add coal" button. If you add 4 at once, it is likely to overheat and you will lose your barrel of coal-tar. The process is fast and may use only 4-5 coal.
 * Grind the quicklime into an empty pot.
 * Use the quicklime and sulphur on the barrel of naphtha to make a Barrel bomb (unfused).
 * Create a strip of cloth by using 4 balls of wool on a loom. A loom can be found inside the building just west of the Falador cabbage patch (South of Falador city).
 * Use the strip of cloth on the Barrel bomb (unfused) to make a Barrel bomb.

Items required: Spade, bow, arrows, rope, plank, tinderbox, a cooked rabbit, and of course, your completed barrel bomb.
 * Head to the catapult north of the Tyras Camp (through the Underground Pass again).
 * Feed the guard a cooked rabbit. Use the bomb on the catapult.
 * Talk to Lord Iorwerth.

The Lord's message

 * Attempt to enter Ardougne Castle.
 * Talk to King Lathas.
 * Quest complete!