Here's how I'd implement reactive/defensive counter spelling:
1: like evocation blocks, you can use them reactively
2: you can't use both a reactive counterspell
and a dodge or a reactive evocation defense at the same time. However, preexisting blocks, and 'instant' enchanted items defense can also be used.
3: the counterspell opposes the
power and not the control of the opposing spell, so it's easier to counter high control, low power spells than it is to block them.
3a: partially effective counterspells reduce the damage from spells. They also work for everyone targeted by the spell, so, for example, if you countered power 4 zone wide effect with a power 3 counterspell, everyone in the zone would be facing a power 1 attack.
4: reactive counterspells must be made 'blind', without the game mechanical knowledge of what the power of the incoming spell is.
5: they cause stress like other forms of evocation.
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My opinion is that if you allow reactive blocks for casters you should allow them for everybody (which would basically mean anyone could defend with any skill they want to and call it a block).
Anyone who pays the points can have a defense with whatever skill they can justify. Evocation and channeling are not free. It's like saying that anyone should be able to use a motorcycle as a melee weapon. The poeple who can already paid for the strength power to let them do that; it was 'free'.