Since spell damage may be reduced or outright repelled, it’s important to understand how binary spells work. Binary spells are spells like STTO, DEST, SPBI, and HYIM which trigger a specific effect on a single creature or group of creatures with a binary outcome (succeed or fail). Also discussed here (in part) are interesting aspects of quasi-offensive spells like FLRE, DISP, and DIIL, as well as team effect spells like YMCA, MACO, and LEVI. A basic understanding of saving throws is a prerequisite to understanding this section.
Fair Warning – the documentation on this subject is incomplete and/or ambiguous. The game mechanics may vary widely among versions (MSDOS vs Amiga, etc.). My testing of these mechanics is ongoing.
Instant Death/Capture Spells
The general rule for instantly killing or capture spells like STTO, DEST, and SPBI is that the victim must have a saving throw that is 4 or more higher than the caster to repel the spell. There is no half-damage option as the spell has a binary outcome. I also assume (which is a huge assumption) that there are no other rules that determine the outcome. This is a questionable assumption given the substantial redundancy between STTO and DEST.
DEST Examples
A level 13 Magician has 18 luck and no Luckshield, yielding a saving throw of 13. The Magician casts DEST at a Xorn, which rolls a saving throw of 9/13/16 (min/avg/max). In this particular case, the Xorn rolls a saving throw of 16. Because the Xorn’s saving throw is only 3 higher than the Magician, the Xorn dies. For all possible saving throws, the Xorn cannot roll high enough and dies.
A level 13 Magician has 18 luck and no Luckshield, yielding a saving throw of 13. The Magician casts DEST at a Greater Demon, which rolls a saving throw of 13/17/20 (min/avg/max). The Greater Demon must roll a 17 or higher to survive. If it rolls a 13–16 it will die. If it rolls a 17–20 it will live. The Magician therefore has a 50% chance of killing the Greater Demon with a single DEST.
A level 13 Magician has 18 luck and a Luckshield, yielding a saving throw of 15. The Magician casts DEST at a Demon Lord, which rolls a saving throw of 15/19/22 (min/avg/max). The Demon Lord must roll a 19 or higher to survive. If it rolls a 15–18 it will die. If it rolls a 19–22 it will live. The Magician therefore has a 50% chance of killing the Demon Lord with a single DEST.
A level 14 Magician has 18 luck and a Luckshield, yielding a saving throw of 16. The Magician casts DEST at a Demon Lord, which rolls a saving throw of 15/19/22 (min/avg/max). The Demon Lord must roll a 20 or higher to survive. If it rolls a 15–19 it will die. If it rolls a 20–22 it will live. The Magician therefore has a 5/8 chance of killing the Demon Lord with a single DEST.
Presumably, spells like SPBI and STTO have identical outcomes in the above examples. However, this is a big assumption.
Group Targeting Spells (e.g., HYIM)
I am currently unaware of the mechanics of spells like HYIM. Many related spells are bugged in the MSDOS version, and they’re not particularly useful for speed runs or farming. They may be useful in the final battle against Mangar, but since they only work on a single group in the next round, the opportunity cost of casting them is prohibitive.
Quasi-Offensive Spells (e.g., DISP and FLRE)
In general, beneficial spells like FLRE and DISP which counter an enemy spell or effect (like poison and possession) will always succeed without respect to the level or saving throw of the enemy caster. FLRE, FLAN, and REST will always cure poison, no matter the source. DISP will always dispossess a hero, no matter who cast the spell.
The mechanics of DIIL are unclear, but it will always, and without fail, identify any doppelgangers or mimics in a party.
Other Spells
Many other binary spells like YMCA, MACO, and LEVI, etc. will always succeed, with the sole exception being when cast in an anti-magic room. Light spells can be cast in, but are also then extinguished by, dark rooms.
Anti-Magic Rooms
Anti-magic rooms generally stop all casting by all casters, character or creature. However, it’s unclear to me if this also includes breath attacks, doppelganger ranged attacks, and doppelganger entries. I believe it does but needs testing. Anti-magic rooms do not extinguish light.
Resources:
Some suggested resources on this subject are: