Magic

In my regular group we have been using Arcane Stress for a while now. I guess since 2011. Yikes.
It works great and does what I want it to do, which is make magic a bit more powerful, weird, and dangerous while changing as little as possible of the regular BX Vancian magic system. The way it works is Magic-Users cast spells just like they normally would in your old school system of choice. If the want to cast any more than they normally would be able to, they can try but they've got to roll a D6 on this table:

1d6	Spell result:
1	Fails. Caster is knocked out d4 turns.
2	Fails.
3	Mutation. Roll on the Spell Mutation Table.
4	Cast at ½ strength.
5	Cast. M-U ages d6 months per spell level.
6	Cast at x2 strength. M-U looses d4 HP per spell level.

Actually some where along the line in the hardcopy I use in game I penciled in some changes:

1d6	Spell result:
1	Fails. Caster is knocked out d4 turns.
2-3	M-U is embroiled in a mystical melee with unseen forces. Roll again next turn.
4-5	Mutation. Roll on the Spell Mutation Table.
6	Cast at x2 strength. M-U looses d4 HP per spell level.

This system is fun because in truly dire situations Magic-Users can take action (magic flavored action) and it might make things worse or it might spectacularly save the day. I changed the table to make mutations more common (because they're fun) but I think the whole idea could be tweaked to be more fun. In my group, when a wizard casts via Arcane Stress it pretty reliably makes the situation entertaining because it is so dangerous/random, but really it is a rarely used last resort.

I feel like it could be changed to make magic more powerful, weird, and dangerous. I've been thinking about this lately because of Brendan's post on Catastrophe Magic. This is a pretty slick roll to cast system. This takes us a bit further from Vancian magic, but it does more to make magic chaotic and interesting without making too much extra work for the player or the DM.

	13	11	9	5	3	1	*	*	*	*
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1

The left hand column is the D20 roll.

The top row is the level of the caster.  

The results break down like this:

	Principal Intensified
	Principal
	Principal and Corollary
	Corollary, Decline and Delay
	Catastrophe

Those are all per Brendan’s descriptions except “Corollary, Decline and Delay” which is corollary and decline plus the spell will go off at the end of the following turn.  After a “Corollary, Decline and Delay” is rolled the MU moves to the next column the next time a spell is cast.  

I’m going to think about this more but I think it could add a bit of interest and make casting get increasingly risky which sounds fun.