Nightmare / Phosani's calculation

Each unique's listed rarity (e.g. 1/750 for the Inquisitor's mace) is the item's real per-kill chance, taken live from the wiki's drop table. On Phosani's Nightmare (solo) every roll belongs to you, so that rarity is used directly.

On the regular Nightmare (5-man), that same per-kill chance is for the whole party — when it hits, only one random party member (weighted by damage) actually receives the item. So your personal expected kills to land it is the listed rarity × 5 (e.g. 750 × 5 = 3,750 kills), which is what the time estimate on this page uses.

Is PNM actually faster for uniques?

Yes — Phosani's Nightmare (PNM) is substantially better for farming uniques, even compared with a regular 5-man Nightmare team. The important distinction is uniques per player, not just the boss's raw drop table.

• Regular Nightmare's unique roll is effectively contested between the 5 players (weighted by damage), so your personal chance per kill is much lower than the ~1/66.7 "someone gets a unique" chance.
• PNM is solo-only, so every unique roll belongs to you. Jagex specifically tuned its rates to make obtaining uniques roughly 20% faster than efficient regular Nightmare, accounting for faster kills and less downtime.
• The 2024 Project Rebalance made Nightmare uniques 50% more common overall, for both variants.
• Regular Nightmare does get a small bonus roll chance as the team grows beyond 5 players, but it's weak — only +1% per extra player, capped at 80 players (+75%). Going from 5 → 10 players doesn't come close to doubling your odds.

Bottom line for the unique grind: PNM >>> 5-man Nightmare. A 5-man team is mainly attractive because it's chill, social and easier — not because it's efficient for your own Inquisitor/orb.