Spellbook of protection

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spellbook of
+   protection   Light green spellbook.png
Appearance random
Abundance 1.83%
Base price 100 zm
Weight 50
Turns to read 3
Ink to write 5–9
Spell type clerical
Level 1
Power cost 5 Pw
Direction non-directional

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 granted by priests or praying.

AC granted

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

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

or

1 + \lfloor \log_2 (\text{XL}) \rfloor - \left\lfloor\frac{\text{prior spell AC}}{4-\left\lfloor\frac{\text{other AC}}{10}\right\rfloor}\right\rfloor

where "prior spell AC" is the AC already gained from castings of the protection spell; "other AC" is 10 minus your AC but for the spell, including all armor bonuses and divine protection; 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

Protection is extremely useful when you are taking many hits from many monsters, or when fighting "out of depth" monsters which you are otherwise unprepared to fight. In these cases it can be the difference between life and death. If you see a difficult monster nearby and are willing to fight it, casting protection before engaging it is always a good idea.

In addition, since it is a level 1 clerical spell, you can use it to train up that spell school.

References

  1. cast_protection in Spell.c
  2. u_init.c in NetHack 3.4.3, line 654


This page is based on a spoiler by Dylan O'Donnell. The original license is:

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This page may need to be updated for NetHack 3.6.0.

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