This writeup is intended to be mobile-friendly as the UI can make switching difficult. Thus, there will be a focus on limiting the amount of switching necessary, from gear to prayers. Using a web browser or PC will help, as you will be able to swap gear much faster, but is not required. Using a PC will allow you to switch prayers, gear, or spells to optimize DPS, but these will not be discussed in depth.
Event mechanics are explained on the Impending Darkness Event page. In summary, you will need to clear 4 Slayer areas a total of 5 times to complete the entire event. At the end of every area cleared, a harder Boss monster will spawn - it is simply a regular monster with extra combat modifiers. At the end of every round of 4 slayer areas cleared, Bane will spawn. You will have time to swap gear before starting the fight with Bane.
Setup
Equipment Sets
You will need to equip each outlined set as you will be switching between equipment sets frequently.
Gear is DR-focused for both mobile- and Hardcore-friendly completion. The rewards given while clearing the event are not required but may help with DPS if you so choose to use them. The Maximum Skillcape is non-negotiable, as fights take place in Slayer areas.
Slayer Negation Sets
This is the most important set and is targeted for Unhallowed Wasteland. You can use this set for the remaining Slayer areas, but it will be helpful to swap to God Armor for better bonuses once completed.
Melee
If you choose the immune to stun modifier, switch off Fighter Amulet.
The God Armour sets can be used for all Slayer areas beside Unhallowed Wasteland. Being mobile-focused, an emphasis on DR options is preferred. Thus Terran and ELS are recommended, but can be swapped for Ragnar and DPS shields respectively if you feel comfortable.
Melee
If you choose the immune to stun modifier, switch off Fighter Amulet.
Maximize combat-focused Astrology bonuses. However, the difference is relatively small, so don't focus on getting optimal bonuses.
Modifier Tier List
Listed from best (choose it) to worse (avoid it). You can start and stop the event when beginning to fish for a decent first modifier. If you receive choices of modifiers or a series of modifiers after a few rounds that are completely awful, you can always restart the event for better RNG.
+100% chance to ignore Stuns and Freezes, Immune to Sleep
+20% Maximum Hit, +20% Accuracy Rating
+2000 Hitpoints, -10% Maximum Hit
+20% Maximum Hit, -10% Maximum Hitpoints
+50% chance to apply poison when hitting with an attack, 10% Maximum Hitpoints
+15% Damage Reduction, +15% Global Evasion
+50% Chance to apply burn when hitting with an attack, +50% chance to apply frostburn when hitting with an attack
+100% chance to ignore Slow effects, -5% Attack Interval
+10% Maximum Hit, -10% Attack Interval, -10% Global Evasion
+15% chance to apply affliction when attacking, +15% Lifesteal
Slayer Area Effects are 5% Stronger
Heal 4% of current Hitpoints every 2 turns, 40% of Maximum Hit added to Minimum Hit
Area Order
The recommended order of areas to clear is as follows:
You can clear in any order you wish, but the recommended order minimizes gear swaps and optimizes Bane fight locations. It is strongly recommended to never clear Unhallowed Wasteland and Dark Waters last.
Fight Details
Prayers
Prayer choice is different for each Slayer area. Prayers are listed in priority order and are recommended this way to reduce the need to change them on mobile as it can be difficult given the UI.
Prayers for Bane fights will be Protect from combat style and Battleheart.
Protect from Melee is required for any Greater Skeletal Dragons. Protect from Magic is recommended to reduce Cursed Lich lifesteal but is not required. Stone Skin will help with Spiked Red Claws. If you feel comfortable, your second Prayer can be Battleheart.
Eating Patterns
Impending Darkness Event takes more attention than Into the Mist. High healing food such as Whale (Perfect) is highly recommended. Pay attention to all attacks throughout the event, and eat to full whenever in doubt. Listed below are the high priority monsters and attacks to watch out for.
Being stunned by a Bone Crush, the -10% DR debuff from Spiked Tail Whip, affliction stacks, plus any additional modifiers makes this pretty dangerous. If you have full debuffs (stun, -10% DR), Eat to full.
These notes are strictly for the monsters in each area. Fire Surge is the recommended spell. Anguish III and Surge II are not required but do provide better DPS; however, they will have to be selected every time you swap to your Magic set.
Swap each set for Combat Triangle advantage. Incinerate and its burn can be used to help quickly DPS each melee UW monster if you wish, but Ocean Song and normal magic works fine.
Bane Fights
Bane's attack style randomly rolls when he spawns. If you don't want to fight with a particular style, you can run - do not Stop the event as this will reset all progress - and reclear the slayer area to roll a new Bane spawn.
Melee Bane
Probably the most annoying style as the modifiers stack up. Nothing special, just let Tidal Edge do the work as you pay attention to his attacks.
Ranged Bane
Decent fight due to the Dragon + Centaur synergy adding a lot of extra damage. Nothing special, as Shockwave is a strong weapon.
Magic Bane
Probably the easiest style as you can Incinerate the entire fight for guaranteed burn and damage. You can switch to any normal spell and start relying on Ocean Song as the burn damage will drop off around half HP if you want, but is not really necessary. You can use normal Magic and Ocean Song the entire fight too. Using Incinerate the entire fight allows you to use Damage Reduction Potions instead of Diamond Luck as your hits are already guaranteed at 20k+ Accuracy Rating.
Slayer areas a total of 5 times to complete the entire event. At the end of every area cleared, a harder Boss monster will spawn - it is simply a regular monster with extra combat modifiers. At the end of every round of 4 slayer areas cleared, 
Bane will spawn. You will have time to swap gear before starting the fight with 
Hardcore-friendly completion. The rewards given while clearing the event are not required but may help with DPS if you so choose to use them. The 
Maximum Skillcape is non-negotiable, as fights take place in Slayer areas.
Unhallowed Wasteland. You can use this set for the remaining Slayer areas, but it will be helpful to swap to God Armor for better bonuses once completed.
Fighter Amulet.
Slayer Helmet (Master)
Slayer Platebody (Master)
Terran God Platelegs
Terran God Boots
Tidal Edge
Earth Layered Shield
Fury of the Elemental Zodiacs
Hunter's Ring
Terran God Gloves
Whetstone
Slayer Cowl (Master)
Slayer Leather Body (Master)
Aeris God Platelegs
Aeris God Boots
Shockwave
Deadeye Amulet - 
Aeris God Gloves
Jadestone Bolts
Slayer Wizard Hat (Master)
Slayer Wizard Robes (Master)
Glacia God Platelegs
Glacia God Boots
Ocean Song
Elite Amulet of Magic
Glacia God Gloves
Prayer Scroll or 
Wizard's Scroll
Terran God Helmet
Terran God Platebody
Aeris God Helmet
Aeris God Platebody
Glacia God Helmet
Glacia God Platebody
Diamond Luck Potions for the entire event.
Dragon + respective Combat style for each set.
Astrology bonuses. However, the difference is relatively small, so don't focus on getting optimal bonuses.
Dark Waters last.
Prayer choice is different for each Slayer area. Prayers are listed in priority order and are recommended this way to reduce the need to change them on mobile as it can be difficult given the UI.
Shrouded Badlands
Battleheart
Perilous Peaks
Protect from Ranged
Stone Skin
Rokken can be scary, so 
Protect from Melee
Protect from Magic
Greater Skeletal Dragons. 
Cursed Lich lifesteal but is not required. 
Spiked Red Claws. If you feel comfortable, your second Prayer can be 
Impending Darkness Event takes more attention than 
Into the Mist. High healing food such as 
Whale (Perfect) is highly recommended. Pay attention to all attacks throughout the event, and eat to full whenever in doubt. Listed below are the high priority monsters and attacks to watch out for.
Ku-tul
Fire Surge is the recommended spell. 
Anguish III and 
Surge II are not required but do provide better DPS; however, they will have to be selected every time you swap to your 
Magic set.
Centaur synergy adding a lot of extra damage. Nothing special, as 
Incinerate the entire fight for guaranteed burn and damage. You can switch to any normal spell and start relying on 
Damage Reduction Potions instead of 
Combat:
