While attack is accuracy, and all the other combat skills are self explanatory, the point of slayer is to know how to deal with creatures that are unconventionally hard to kill, adaptive, and have few weaknesses. Even if you have 99 all combat if you have 1 slayer you won** know how to deal with slayer monsters properly.
I believe the original slayer monsters were specially corrupted by xau-tak to be very hard to kill and in many different varieties so expelling all of them would be specially difficult. Thus xau-tak would always surivive in some niche and be present in some place. The rest of the slayer monsters that obviously don’t belong to xau-tak like Airut and spiritual mages happen to fall into slayer because they are hard to kill and have specific weaknesses.
With slayer being one of the main themes around Xau-tak, it makes sense that the player, and all adventurers may be one of the most important, if not only methods of defeating xau-tak and his army.
Locations and xau-taks role in them.
Relekka dung, Polliv dung, and lumbridge swamps, Abyss ((chthonian)), Daemonehim
These are all places were slayer creatures, specially the original ones can all be found (except abyss but that’s specific).
These locations I believe are also clues to Xau-taks strategy and origins on Runescape. The things these locations have that are very particular are the slayer monsters, their interdimensional elements, and their ties to the stone of Jas (OOOH)
Abyss:
The abyss is the glue that holds together realms and contains abyssal demons. Now abyssal demons are demons and also Chthonian which have clear lore behind them as the 2nd rulers of infernus, you can read up wiki on them. We don’t really know the origin and earliest moments of Infernus, but I believe the chthonians were early corruptions by xau-tak on infernus.
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Valor, Vindication, Vravery! For V!