You can kill The Nightmare to receive the inquisitor's armor set, the inquisitor's mace, the nightmare staff, and the 3 orbs that can be combined with the nightmare staff. (harmonised orb, eldritch orb, & volatile orb)
The Nightmare can be soloed by using
+nightmare solo, or massed by using
+nightmare mass. (Masses require multiple players to join.)
While killing The Nightmare, your minion has a chance to die. This chance is affected by your kc and gear. Your chance of death has a maximum of 90%, and a minimum of 2%.
The time required to kill The Nightmare is also affected by your kc and gear.
At <10 kc, there is a 15% penalty to time, from 10-24kc, the penalty is 5%, and from 25-49 kc, it is 2%. From 50-99 kc, you get a 2% bonus, and this bonus increases to 4% at 100+ kc.
Having less than a certain % of the maximum melee strength will also give a penalty towards your time. At <82 melee strength, this penalty is 6%, from 83-102 melee strength, it is 3%, and from 103-123 melee strength, it is 2%.
If you are using a weapon that has less crush attack than a Zamorakian Hasta (75), there is a 5% penalty to time.
There is also a boost applied for having the inquisitor's armor set and the inquisitor's mace equipped in your melee setup. Each piece provides a 2% boost in solos, and a 0.5% boost in masses. If you have all 4 pieces equipped (mace, hauberk, greathelm, and plateskirt), an extra boost of 2% for solos, and 1% for masses is applied. This totals to a 10% boost for full inquisitor's in solos, and a 3% boost in masses.
Slash defense and crush attack are also considered when killing The Nightmare, and affect food use.
Having an Elysian Spirit Shield equipped in your melee setup will also reduce food usage by 17.5%, however this is outweighed by the food reduction of using an elder maul or a dragon defender.