It seems to me that in 0.5.2 Extension is quite overpowered, especially in combination with Necromutation.
It is possible to cast Necromutation in the late game even with 4 or less skill in Necromancy and Trasmutation, by wearing a staff, 2 rings of wizardry and intelligence boosters. It will have a significant failure rate, but you can just try until it succeeds. Once it succeeds, it will reset satiation and give you foodlessness and you can then cast Deflect Missiles, Insulation, See Invisible, Control Teleport, Flight, Swiftness and Ozocubu's Armour. All these can be learned, cast and then forgotten with Selective Amnesia.
Forescry has only a 30 turn cap, so indefinite extension is possible but with glowing problems. Ring of Flames cannot be extended indefinitely
At this point, you can just cast Extension every time an effect expires, using free Sif Muna channeling (due to Necromutation foodlessness) to get the 5 MP to do so if necessary, and make all effects permanent.
You will get some glowing but not in the mutagenic range unless really unlucky. Even then, mutations will only rotting since you are in lich form.
Using Haste is somewhat problematic since Extension will extend Haste too. However, if Extension is cast twice just before Haste while wielding a staff of enchantment, this problem is minimized
Emergency Hastes will probably however cause some effects to fall apart.
Slow can be eliminated by wearing an amulet of resist slowing, which will cause Extension to not extend Slow.
Thus at the cost of 9 spell slots (for Extension and Selective Amnesia) you get:
- rC+ rN+++ rElec rPois SInv cTele Flight Swift DMsl MR+50
- AC+7 to AC+20 with 4+Ice/3 (Ozocubu) +3+Necro/6 (Necromutation)
- Occasionally EV+8 with Forescry
- Torment immunity
- Immunity to mutations (they cause rotting instead, and potions of heal wounds are much more plentiful than cure mutation)
- Foodless (free Sif Muna channeling! and no need for an amulet of the gourmand)
- Necromancy enhancement
With (somewhat) significant downsides:
- Vulnerable to dispel undead (the only serious problem, but only Shadow Fiends, Gloorx Vloq, necromancer ghosts and some Pan lords use it)
- Emergency Haste is likely to cause rotting/mutation issues
- Cannot use regeneration
- Cannot use Borgnjor's Revivification and Death's Door
And insignificant downsides:
- Cannot quaff potions (but you can just use a wand of healing for combat use and end the transformation to heal rotting)
- Cannot cast Alter Self, Berserker Rage, Blade Hands, Borgnjor's Revivification, Cure Poison, Death's Door, Dragon Form, Ice Form, Alistair's Intoxication, Passwall, Regeneration, Resist Poison, Spider Form, Statue Form, Stoneskin, Summon Horrible Things, Symbol of Torment, Tame Beasts
[the only useful ones in the late game are possibly Berserker Rage, Borgnjor's Revivification, Death's Door, Stoneskin and Summon Horrible Things]
- Cannot cure poison due to poison arrow
- Cannot eat mutagenic food
- Cannot wield holy wrath
Is this behaviour intended?
If so, it may make sense to make it less annoying (right now you need to set force_more_message, travel_stop_message, and manually cast Extension every time you get a prompt).
Otherwise, maybe something should be changed.
For instance, rolling spell success with Extension would prevent Necromutation with no necromancy skill, while increasing glowing to only allow a limited number of consecutive extension would prevent permanent effects without spell slots.