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Mike’s D&D Blog has an interesting post on a terrain effect from past editions that’s fallen by the wayside in the current edition: anti-magic zones. I think that, with anti-magic zones, you’d want to be very careful not to make them too big. A 2×2 area on the battlefield would be fine, and you might be able to get away with a little more, but probably not much larger.
One of the things you have to remember is that, if all you’re stuck with is basic attacks, your turn is likely to be a little on the boring side. In addition, if you happen to be one of the many classes that do not rely overmuch on Strength (for melee basic attacks) or Dexterity (for ranged basic attacks), your basic attacks are not going to be very effective. Overusing an effect like this could make an encounter both longer and less fun if not employed carefully.
As an alternative, I’d put rather use a Wild Magic Zone, and I might even key it to a particular power source. This way, rather than simply nerfing a character in the zone, you give them a choice: go with the basic attack that reliable but not that effective, or gamble a bit with a power that might be a lot more effective than normal, a lot less effective, or possibly even harmful to the party. A zone that, for example, randomly either increases or decreases the size of bursts and blasts, or causes attacks to occasionally target random creatures within the zone rather than their intended target, might be considerably more fun than a zone that simply shuts down your options.
I actually used something similar to this effect in my most recent session. The first encounter took place in a room in which there was a conduit to the Far Realm, which was bending time and space in strange ways. It had two effects. First, any burst or blast in the area had a 25% chance of increasing its size by 1d4 squares, as well as a 25% chance of decreasing its size by 1d4 squares. Second, any ranged attack had a 50% chance of targeting a random creature in the room. In this particular case, the roll results favored the PCs more than the monsters, but it added the element of unpredictability that I wanted.
Related posts:
The wild magic zones were a nice addition to the session. Although, I may be saying that because the rolls went my way and gave me good mojo. I’ll reserve the right to talk smack about this rule if it goes against me…
Yeah, I think most of the time with the ranged attacks I forgot to roll, and with the burst attacks it almost always helped you guys out. I’ll have to re-use this mechanic later, and remember to use it properly (i.e., make it eff you guys up a bit).
[...] a few things that I’ve mentioned on this blog. The first encounter included a variant of wild magic rules; unfortunately, I forgot to utilize them as often as they should have been, but when I did it [...]