For at least some of the versions of the game, any effective spell level of 60 or more "overloads" the calculations in the game engine, and the damage drops dramatically - IIRC to about 8 hit points. ((You'd need Bones or somebody to give you a mathematically rigorous explanation of the precise reasons.)The effect of Guinevere is to multiply your effective skill level in both Dark (and Light) Magic by 1.5, so a skill level of 26 becomes effectively 39. This effect stacks with the effect of a Ring of Dark Magic, which also multiplies the effective skill level by 1.5, so the effective skill level of 39 becomes 58 (the calculation gives 58.5, but this is rounded down). This is under 60 and so is OK.
However, a similar calculation applied to a skill level of 27 gives a first increase to 40 (actually 40.5, but this rounds down as before) and and a second increase to 60. Bang - you've just fallen off the edge of the cliff, and your character's effective damage has just dropped to something negligible.
Try it - it's easy enough to change skill levels with a savegame editor. You may be lucky, and yours may be one of the games which is not affected by this. But I know mine is.
