Bestow Curse is the example of a spell with a powerful but complete random effect and this kind of mechanic doesn't work very well in a single-player game. It will correctly fit in a real-life session of Dungeons and Dragons with a human DM - is a high-risk high-reward magic and once the roll is done, the Dungeon Master decides what happens appropriately and the session continues. But in a single-player PC game, the user can simply reload the game until the Lady Fortune smiles and get the best result. There are no profound deliberations or proper tactical considerations, simply roll the dice and if you don't like the outcome, reload the game until you win. The 50% effect could have some utility in the Mass Bestow Curse spell, given that it affects several enemies at the same time but, again, is tossing a random shot for the best option. Even if the effect continues active in subsequent turns the mechanic of casting a spell, tossing a coin and hoping for the best is not adequate for a heavy tactical game with lots of options. The spell was designed to be played with a Dungeon Master acting as an arbiter, avoiding obvious abuses and the mechanic of high prize or nothing is not really suitable for tactical games.
Also, the spell has been weakened compared with the AD&D, D&D 3.5 and Pathfinder versions: in those rulesets, it has several options available to choose. These are missing:
-> -6 decrease to an ability score (minimum 1).
-> -4 penalty on attack rolls, saves, ability checks, and skill checks.
In the Pathfinder ruleset there are listed several possible alternatives to the curse effect with a similar power level, in case you require more variety:
1) -> When the victim is adjacent to the area of a damaging spell or spell-like effect, the area expands to include the victim.
2) -> The victim can’t heal naturally, and magical healing cure the victim by only half the usual amount (minimum 1 point). Fast healing and regeneration are likewise halved.
3) -> Whenever the victim takes damage, he is staggered for 1 round.
4) -> The victim is plagued by cacophonous sounds and strobing lights that only she can hear and see. She is distracted (+5 to Perception DCs), cannot take 10 on skill checks, and must succeed at a concentration check (DC 10 + spell level) to successfully cast spells.
5) -> Anytime the victim picks up or retrieves an object (including drawing a weapon or ammunition), there is a 50% chance that he immediately drops it. If ammunition is dropped, the attack being made is lost. Any remaining attacks in a full attack action may still be attempted.
By the way, the Staggered condition mentioned in the third option could represent a very welcomed addition to the game. In few words, a staggered creature may take a single move action OR standard action each round (but not both, nor can he take full-round actions).
I think the spell should be changed; don't know if the most convenient option is to add the other two original effects to give more options to the caster when casting this spell and/or to get rid of the random effect completely or including some of the optional effects mentioned in the Pathfinder ruleset. The "minus six decrease to an ability score" options feels a little weak compared with the level-1 spell "Ray of enfeeblement" effect. Would it be more adequate to add a second negative malus applied to a different ability? But only a -2, for example?
The other five optional ones can be included to the game in future updates in new curse spells (long-term objective, there are others priorities). The third and fourth ones sounds extremely useful for more tactical options in the game.
The 8-level version of this spell (Mass Bestow Curse) should be modified according to the changes done, obviously.
Thanks for reading!