Magic works better in the underworld and this encourages powerful wizards to begin construction of dungeon complexes. Magic working better in the underworld can have a number of different ways in which that can be presented.
- Magic is Weak on the Surface But It is Stronger in the Underworld. In this arrangement magic spells only function past their base effectiveness deeper and deeper into a dungeon. All those variable effects that make a spell more powerful only work deeper and deeper into the dungeon.
- Magic is Tricky on the Surface and Easier to use in the Underworld. In this arrangement spells are not always cast successfully. The deeper one goes into the underworld the more likely spells are to work. Magic-Users don't lose a spell when they aren't cast right, they just don't work and the magic-user has to try again...
- Magic Only Works in the Underworld. In this arrangement Magic only works in the underworld or perhaps only works when deep enough in the underworld. 1st level spells only work on level one or lower, 3rd level spells on the third level or the dungeon or lower.
- Magic Spells are Quickly Recovered in the Underworld. In this setup magic spells cast by a magic-user instantly return to the caster after an appropriate period of rest and don't require lengthy or bothersome spell memorization/preparation after that (so long as they are the same spells used yesterday). It's probably a good idea to have this feature keyed to dungeon level so 1st level to have those 1st level spells restored, 2nd level for those 1st and 2nd level spells (and so on). The period of rest can be as long as the DM decides is appropriate for the campaign or dungeon. If the rest before recovery is brief Magic-Users are more powerful but it also encourages parties to stick around in the dungeon and clear out a safe area to rest,
The creation of magic items can likewise be tied to dungeon level. That help's to explain what dungeons are doing under castles and wizard towers in the first place. This can be used in a number of different ways.
- Magical Earths and Minerals. In this setup magical earths and minerals required to fashion magicl aitems of all sorts are mined deeper and deeper within the underworld and lose their effectiveness if brought above ground for too long before being used in the construction of a magical item. You could key such materials to levels or a range of levels. Regional variations in what earths and minerals are available will also give cause to dungeons in multiple regions.
- Magical Forges Must be in the Underworld. In an underworld where this is the case a forge capable of fashioning +1 magical weapons (or equivalent items) may be no nearer the surface then the 2nd dungeon level and +2 magicla gear may be fashioned no higher then the 4th level. This would give ample reason to dig deep to construct workshops capable of fashioning the required magical gear.
- Clerics Worship Dark Gods with Deep Shrines. Clerics are required to make a pilgrimage to secret temples and shrines deep in the underworld before they may enjoy the abilities of earned with experience points. Sure a cleric may have enough experience points to be 3rd level but until that cleric offers sacrifice in a shrine on the 3rd level of the underworld there will be no benefit gained.
- The Dead Must Rest Deep in the Underworld Before Being Raised. For any form of magic that brings one back from the dead may be effective they must rest in a sarcophagus deep in the underworld. This can tie to the level of the person to be restored to life, cleric level, or spell level as the DM feels is appropriate for campaign or dungeon. Time spent in such a sarcophagus shouldn't work against the success of such vilification and there may be special sarcophagi that extend the time the remains can be restored.
- Portals to the Afterlife. There are portals to the afterlife in the underworld and these very portals may be opened and the glooms may be traveled to bring soemone back from the dead. The spells may act as a key, or only function when the spirit has been dragged back from the beyond.
What of those poor classes with magic tied to the wilderness on may be asking: Rangers, Druids, and the like might not be bound to the dungeon realm and will have greater relative power if allowed in a campaign with a wyrd underworld but mayhaps they must visit stone circles deeper and deeper in the wilds to gain access to their magics in a campaign with a wyrd underworld.