Campaign-Changing Magic Items

The PCs in DFRPG Arden Vul have found a few unexpectedly powerful magic items that have significantly changed the campaign.  If I had written Arden Vul, I would not have put any items that powerful that close to the surface.  As a teenager I read all the Gygaxian advice from the 1970s Dragon magazines about avoiding Monty Haulism, because once power spirals out of control the campaign is doomed, and I've tried to be cautious about giving out too many powerful items ever since.  But maybe it's different for items that aren't the +5 Weapon of Effortless Killing or the +5 Armor of Completely Ignoring Danger.  Maybe Richard Barton knew what he was doing.

Larel's Sack

Larel's Sack is a Bag of Holding.  Not even a particularly big Bag of Holding.  It can also create small items, with limitations.  (Though my PCs haven't really used that power much; they just use it as a Bag of Holding.)  And they found another, larger, Bag of Holding later, so Larel's Sack doesn't feel that powerful anymore.  I think the first method PCs find to significantly increase carrying capacity is a big deal (as it means they can get heavy treasure out of the dungeon, which increases their loot and thus their XP if you give XP for loot), but additional ways of carrying more stuff give diminishing returns, once you can usually carry all you find.

Larel's Cloak

Larel's Cloak gives Invisibility, which is either amazing or not a big deal, depending on whether you already have a mage who can reliably and repeatedly cast Invisibility.  It also gives reaction bonuses, which is either amazing or not a big deal depending on how often NPCs are actually making reaction rolls.  If the GM makes reaction rolls in secret then players might underestimate the value of reaction bonuses, since they can never be sure if that NPC was always going to be friendly, or if their reaction bonus saved them from a fight.  On the other hand, if the GM mostly determines NPC reactions by fiat, maybe the bonuses don't help much.  (You can't use random reactions for every single NPC, or the beautiful nymph face character just charms her way past every challenge and the other PCs are just there to carry the gifts from her adoring fans.)

Larel's Pin

Larel's Pin is a really good magic stiletto.  It has bonuses to skill and damage, and a chance to slow opponents it hits.  In games where every weapon is about the same, that's a really good weapon.  In games like GURPS where different types of weapons use different skills and do different amounts of damage, knives aren't great.  They don't completely suck, because you can use them in close combat, and you can fast draw them, and you can hide them.  But if nobody puts points in Knife skill because a knife isn't a good a general-purpose weapon as a sword or axe or spear, then nobody's going to get that excited about a magic knife.  I was kind of surprised that my PCs actually chose to sell it, but it was worth a lot of money and they wanted to curry favor with Wicktrimmer, one of the more connected and powerful NPCs in Gosterwick.

Rugs of Instant Access

Teleportation is especially called out as a problem in DFRPG, and the Teleport spell from GURPS Magic is not in the DFRPG spell list.  (There's an advantage in one of the Dungeon Fantasy books to get around this if the GM wants, but it's 100 points, not 1 point [once you have the prerequisites] like most spells, so it's not common.)  The Rugs are a super powerful item because they allow teleportation.  They are also portable, and seem to be reliable.  A portable, reliable teleporter is an extremely valuable thing.  But they have limits.  You can only teleport from one Rug to the other Rug, so you need to get them both in place first.  So you can use them to go between two fixed guarded points, but how many PCs have two fixed guarded points?  If you want to go from one fixed guarded point to random places, someone has to carry the other Rug there first.  If you want to use the Rugs to evacuate the whole party, you're leaving a Rug behind with whatever made you run away, and you might not get it back.  So I think the Rugs are amazing most of the time, but they have drawbacks.  Probably the best kind of magic item.  They change the campaign, but make the players make difficult choices.  Do you sell it to an NPC, do you use it for your commute, or do you use it for tactical movement?  So far the Right for Riches company has gone with tactical movement.