Aehm .... there was a change quite a while ago ...
Slayers dont do double damage ... they add 100% di against fitting monsters. Slayer's count towards the cap of 300% di. The difference comes into work, if someone wears the undead slayer talisman and an undead slayer weapon. I am not shure, if both add 100% (due to the nature of having the same slayer type twice) but if they do, they simply add 200% di to undead. (I've only tested with an undead slayer weapon and a mage slayer talisman ... both work on Liches.)
Yep, that's the way they work.
But with a 300% vs. Monster DI cap, one hits that cap fairly easily, and there's that annoying "lost DI" bug (where it seems that if your DI goes over 300%, the entireity of the last bonus added, not the part over 300% - and/or that Crushing Blow will void most of the other DI bonuses in the process of being applied).
+100% from slayer 1
+100% from slayer 2 (talisman, or old double slayer)
+25% from Stormgrip
+35-50% from weapon
+25-40% from other sources.
= 300% (note that Tactics, Anatomy, and LJ bonuses add in separately, IIRC to the BASE damage for the weapon that is displayed on your status bar)
Makes Crushing Blow (+50% DI on a single attack) a worthless special move, even with only one slayer and using other means of gaining DI (like Enemy of One).
Now, Enemy of One is supposed to be +50%, yet the Devs have swore in the past that EoO + Slayer is at or near max DI. This would only be true if the DI from all other sources is added, then multiplied by 50%, then added - or if the BASE damage (treated as 100%) + existing DI are multiplied such before calculating.
Scenario 1: DI with EoO = Pre-EoO DI x 1.5 (to a maximum of 200% pre-EoO DI)
Scenario 2: (Base damage (100%) + pre-EoO DI) x 50% = EoO bonus, resulting in (with a slayer) at least a 100% DI bonus after modification.
If Draconi or someone similar reads this, want to clarify?