spellbook of
+   protection
Appearance random
Spell type clerical
Level 1
Power cost 5 Pw
Ink to write 5–9
Cost 100 zm
Weight 50

The spellbook of protection provides a useful spell that decreases your AC, but only temporarily. The exact reduction depends on your experience level, the amount of protection already gained from this spell, and your AC before casting the spell. The protection wears off at a rate of 1 point every 10 turns, or 1 point every 20 turns if the spell was cast with expert proficiency.

The spell is unrelated to protection from priests or praying.

AC granted

The exact formula for determining the granted protection[1] is whichever of the following two is more:

${\displaystyle 1 + \lfloor \log_2 (\text{XL}) \rfloor - \text{prior spell AC}}$

or

${\displaystyle 1 + \lfloor \log_2 (\text{XL}) \rfloor - \lfloor\frac{\text{prior spell AC}}{3+\lfloor\frac{\text{other AC}}{10}\rfloor}\rfloor}$

where "prior spell AC" is the AC already gained from castings of the protection spell; "other AC" is your natural AC (gained from donating to priests or by divine protection granted while praying on an altar); and log2(XL) is just the logarithm with base 2, according to the following table:

Experience level log2(XL)
1 0
2-3 1
4-7 2
8-15 3
16-30 4

Thus you will get 1-5 AC on the first casting depending solely on level, and on subsequent castings you will get progressively less depending on your AC and protection level.

XL Other AC Total AC from n castings
1 2 3 4 5 6 7 8 9
1 10 to 1 1 2 3 4 4 4 4 4 4
1 0 to -9 1 2 3 3 3 3 3 3 3
1 -10 to -19 1 2 2 2 2 2 2 2 2
1 <-19 1 1 1 1 1 1 1 1 1
2-3 10 to 1 2 4 5 6 7 8 8 8 8
2-3 0 to -9 2 4 5 6 6 6 6 6 6
2-3 -10 to -19 2 3 4 4 4 4 4 4 4
2-3 <-19 2 2 2 2 2 2 2 2 2
4-7 10 to 1 3 6 8 9 10 11 12 12 12
4-7 0 to -9 3 5 7 8 9 9 9 9 9
4-7 -10 to -19 3 5 6 6 6 6 6 6 6
4-7 <-19 3 3 3 3 3 3 3 3 3
8-15 10 to 1 4 7 10 12 13 14 15 16 16
8-15 0 to -9 4 7 9 10 11 12 12 12 12
8-15 -10 to -19 4 6 7 8 8 8 8 8 8
8-15 <-19 4 4 4 4 4 4 4 4 4
16-30 10 to 1 5 9 12 14 16 17 18 19 20
16-30 0 to -9 5 9 11 13 14 15 15 15 15
16-30 -10 to -19 5 8 9 10 10 10 10 10 10
16-30 <-19 5 5 5 5 5 5 5 5 5

Identification

Monks have a 1/3 chance of starting with a spellbook of protection[2].

Messages

The <air> around you begins to shimmer with a golden haze."
You cast the spell
"The golden haze around you becomes more dense."
You cast the spell while already protected by it
"Your skin feels warm for a moment."
You cast the spell, but can receive no more protection
"The golden haze around you becomes less dense."
The spell is wearing off
"The golden haze around you disappears."
The spell has worn off completely.

Strategy

The spell is best used just before fights with "out of depth" monsters which you are otherwise unprepared to fight.

A routine to fight a difficult monster might be:

Source code references

 This page is based on a spoiler by Dylan O'Donnell. The original license is: Redistribution, copying, and editing of these spoilers, with or without modification, are permitted provided that the following conditions are met: The original contributors to any spoiler must continue to be credited. Any modifications to the spoiler must be acknowledged and credited.