Following completion of Into the Mist, you now have access to Dark Waters, the penultimate slayer area. These 3 mobs drop fragments for the T90 weapons, the highest level, and often most powerful, weapons in the game that are extremely helpful in completing the following slayer area, as well as the Impending Darkness Event. These are extremely long grinds, and require quite a bit of preparation. Each weapon requires 100 fragments, and the fragments have a 2% drop rate. That's 5,000 kills on average.
We recommend acquiring them while fighting each monster as a slayer task, this will require you to turn OFF auto-slayer, and will encourage semi-frequent check-ins. Doing these on task will increase your damage dealt to them from various buffs, namely Slayer Skillcape. This will make the order you obtain the final weapons in effectively random, but if you want to do them in a specific order, we recommend the following:
Additionally, you should also acquire the pet Otto whilst doing this grind, if you do not, continue farming whichever mob you kill the fastest on or off task until you get it.
Item fragments are very rare drops, and you should attempt to sneak in as much item doubling into your gear as possible. Using Combat Sim to measure your Drops/hr with no deaths is highly recommended. It can make the difference in literal days of grinding.
Order
The order you obtain these items in is not particularly important.
While it may seem that using these weapons to farm the other weapons would be beneficial, this is not the case due to the relatively weak power level of Tidal Edge and Ocean Song compared to other available weapons. The only suggestion here is to obtain Shockwave and use it to obtain Ocean Song
Between those two, the required time is roughly equal.
Many people combine the grinding of these weapons with the grind for the master slayer gear. An added benefit is the increased damage given by the Slayer Skillcape and buffs from Astrology while fighting slayer tasks. Another benefit is the ability to buy prayer points, Light Runes, and Foodat a profit with the use of Generous Resupply.
Requirements
There are very few hard requirements beyond the ability to survive and the entry requirement of Slayer Level 90, but there are several skills and items that will make this grind significantly easier, and we would be remiss to not mention them
Ocean Rafting is technically the best dps option. Lava Waterfall Dodge is recommended slightly above it, as you preserve some of your ammo and runes, earn more coins, and have an easier time swapping to skilling to restock.
For Ranged, with Shockwave, it is sometimes possible that Ranged Strength Potion IV is stronger than Diamond Luck Potions. This should never be done without checking Combat Sim for yourself, and should never be done unless the potion is specifically Ranged Strength Potion IV, and not any lower mastery.
Notably for Magic, using use Cyclops and Witch will earn you significantly more coins at a fairly minimal loss of kills per hour. This is most obvious with Cloudburst Staff.
Tidal Edge
To be able to idle Umbora with Magic, you'll need 47 DR assuming you have 1000 HP.
Umbora can be killed with Normal Magic if you have a Cloudburst Staff or an Ocean Song, or with Ancient Magicks at a much slower rate. Note that Cloudburst is the fastest method, even if you have to squeeze in extra DR from Guardian Ring or Prayer
40% slayer area reduction increases your dps against Ku-tul, and therefore, unless it will lead to your untimely demise, at least a Slayer Cowl (Elite) should be used.
While the other two weapons are pretty straightforward in terms of gear, the Shockwave is not.
Rokken is by far the most dangerous and is both willing, and able to kill you if you are not extremely careful in your setup. This monster is able to buff itself to an attack speed of 2.2s, and is able to stun you, which massively increases its damage against you. Your goal here is either to be able to survive the increased stun damage, or to never actually take the stun damage by having an equivalent or faster attack speed than the monster.
You need to use Melee for this grind, and as said before, it is highly recommended that you use an Infernal Claw to complete this. Tidal Edge is too slow to reliably survive Rokken's stun without significant extra work, requiring you to put on more DR that will massively affect your kill time. Assuming you were able to use the exact same setup with no Astrology buffs, Infernal Claw is only 1-2% slower than Tidal Edge. Adding buffs from Astrology, adding the additional DR or attack interval reduction required for Tidal Edge, and using summoning combat familiars invert this difference and make Tidal Edge up to 20% slower with no tangible benefit.
The Fighter/FEZ is the best passive/neck setup, even beating out the item doubling from Chapeau Noir. Note that wearing FEZ is required to wear Master Slayer armor.
One major note is that the Ring of Blade Echoes is marginally faster for fragments/h than all other rings, however, due to its interaction of significantly increasing your use of consumables like Prayer Points and Potions, it is lower on the rankings.
For this set-up to work safely without risk of death, the player's attack interval while in combat with this enemy must be less than or equal to 2.2 seconds, as this means the player is quick enough to avoid taking extra damage when Rokken attacks with the Shockwave special attack. To achieve this, we use a weapon that has a base 2.2s interval, and gather 40% total slayer area negation through Slayer Skillcape and Slayer Helmet (Elite).
Check your setup with the Combat Simulator before engaging Rokken.