I see. The current wording doesn't appear to cover spells that target the Arena or spells cast by Spawnpoints or Familiars. You may need to adjust the wording to include these. Also is the intended wording "cast a spell" or "cast and resolve a spell"?
The ability is not intended to apply to arena spells or spawnpoints/familiars; I wanted to keep it as simple as possible. "cast a spell" is correct, as the wording has the premonition markers (or mana) used as an additional cost of casting that spell.
Overall it feels like it gives the opponent too much control over the Prophets ability to see the future. I would consider some effect that is more straightforward and identifies some type of information about the future that the opponent must reveal. For example, the opponent must declare what spell types were readied during the planning phase. E.g. a Conjuration and an Enchantment. This would be faster to execute each round then placing markers or showing specific cards.
This isn't the entirety of the prophet's ability to see the future, but just an aspect. In this case, the prophet has feelings about the locations where things are going to happen. I agree that knowing what spell types/spells were readied is another excellent future sight power, but that is covered by the Crystal Ball. Placing markers should not take too much time. The prophet's abilities are strongly reflected in his Prophet only cards, namely:
Foresight (the most versatile spell in the game)
Staff of Clairvoyance (the prescient trait is much more powerful on a mage than on a creature since mages can spells)
Crystal Ball (tells you exactly what the enemy mage has prepared)
The Book of Prophecy (makes the prophet's innate premonitions much more binding and prevents the "place premonitions everywhere" tactic)
Another approach is to think about the benefit the Prophet might receive from knowing pieces of the future and design the ability for those effects. For example a set of markers like the Anvil Throne Warlord except each one has a hidden effect. E.g. Avoid Attack, Counter Spell, Take Initiative, etc.
That is a great idea that I may have to incorporate into a card or two. Much like the Anvil Throne Warlord's ability, it would require a separate explanation in the rulebook if used on the prophet's card.
How was it decided that this would be an Arcane trained mage? The ability to see the future seems more appropriate to the Mind school.
Arcane seemed like a better fit to me. I know precognitive powers are often associated with psychic 'flavors', but the predicting the future feels more like pure magic than flavored magic to me; seeing the future has nothing to do with manipulating or reading minds, and windows in time (and things like crystal balls) just seem to fit more in the pure arcane school. Somebody else (can't remember who at the moment) suggested that the holy school might also be a good fit, given the status of prophets in religions; I don't really see him as that sort of prophet, though. This is mostly just a personal flavor preference on my part.
If he were in the mind school, of course, some thought would need to be given to the types of creatures he would have, since any mind creatures would be usable by the forcemasters.
It might also be an interesting design to give the Prophet training in all schools but only to certain levels. E.g. level 2 in all schools. This would reflect the ability to see the future and have an appropriate set of counter cards in the Prophets spellbook at the start of the battle. 
I feel that this is better accomplished through the Foresight spell. Prophets strike me as specialists (he should probably have an anti-training). I see what you are going for, though. Thanks for the suggestions; if I make another iteration I might incorporate some of them.
