Of Fatal Lore's two modes, only one targets a creature (C.R. 601.2b-c, 700.2f). If you start to cast Fatal Lore and the opponent chooses the second mode, Fatal Lore will cost 2 less due to Killian's cost reduction. If the first mode is chosen, the reduction won't apply since the first mode doesn't target anything.
In general, if a spell is modal, as Fatal Lore is, the appropriate modes are chosen before the spell's cost is determined and paid, and the spell's targets are determined only by the chosen modes (C.R. 601.2b-c, 601.2f, 601.2h, 700.2f). This applies to Fatal Lore just as it does to Abrade or Jund Charm.
Also, in the case of Game Plan or Fatal Lore, if you find yourself unable to pay the spell's cost (e.g., even assisted by another player in the case of Game Plan [see C.R. 702.132a]), then the action of casting the spell is illegal (C.R. 601.2; see also C.R. 728.1). But if an action is not illegal, it's not reversed and you must continue with the action. In a sanctioned tournament, however, a player is "not usually allowed to take back an action that has been communicated to their opponent, either verbally or physically" (M.T.R. 4.8). An exception is if a judge decides with certainty that a player "has not gained any information since taking [an unintended] action" and decides to "allow that player to change their mind" (M.T.R. 4.8). See also this thread.
EDIT (Oct. 18): One rule was renumbered in the meantime.
EDIT (Sep. 20, 2023): Edited, including to update rule numberings.
Modes are chosen before targets (because modes can influence what can be targeted, like in this case). The opponent choosing the mode doesn't change that. Since you are the one casting the spell, if the targetd mode is chosen, the cost reduction will apply.
601.5. Some spells specify that one of their controller’s opponents does something the controller would normally do while it’s being cast, such as choose a mode or choose targets. In these cases, the opponent does so when the spell’s controller normally would do so.
Likewise, since targets can influence costs, like in this case, targets are chosen before costs are determined.
You can legally start to cast Fatal Lore even if you don't have the means to pay for its later cost at this time. When costs eventually have to be paid, and you can't, the spell casting is nullified and the game is rewound to right before you started casting (with some actions being excempt from being reset). You may then restart the casting, and since that would result in a loop if the opponent chooses the same mode again, you as the active player will have to abandon that course of action and do something else. If you could somehow cast the spell in your opponent's turn, you can force him to choose the targeted option, because he then has to make the differing choice to break the loop as he is then the active player.
725. Handling Illegal Actions
725.1. If a player takes an illegal action or starts to take an action but can’t legally complete it, the entire action is reversed and any payments already made are canceled. No abilities trigger and no effects apply as a result of an undone action. If the action was casting a spell, the spell returns to the zone it came from. Each player may also reverse any legal mana abilities that player activated while making the illegal play, unless mana from those abilities or from any triggered mana abilities they caused to trigger was spent on another mana ability that wasn’t reversed. Players may not reverse actions that moved cards to a library, moved cards from a library to any zone other than the stack, caused a library to be shuffled, or caused cards from a library to be revealed.
725.2. When reversing illegal spells and abilities, the player who had priority retains it and may take another action or pass. The player may redo the reversed action in a legal way or take any other action allowed by the rules.
724.3. Sometimes a loop can be fragmented, meaning that each player involved in the loop performs an independent action that results in the same game state being reached multiple times. If that happens, the active player (or, if the active player is not involved in the loop, the first player in turn order who is involved) must then make a different game choice so the loop does not continue.
In the case of the Assist ability, you cannot force a player to pay for your spell in this manner, and will have to back out of casting the spell if it cannot be paid for, and do something else after the attempted casting is reversed.
Killian, Ink Duelist
When casting Fatal Lore, will Killian be able to reduce the cost of it, given the opponent chooses the choice that targets?
If he can, can you announce Fatal Lore with three black sources and Killian in play with no way to pay for a fourth mana?
Or do you pay 2BB before the opponent even gets the opportunity to "choose one" as part of casting the spell?
Also, in this case, or the case of Assist and Game Plan, you may be forced into backing out of casting the spell after announcing it, right?
In general, if a spell is modal, as Fatal Lore is, the appropriate modes are chosen before the spell's cost is determined and paid, and the spell's targets are determined only by the chosen modes (C.R. 601.2b-c, 601.2f, 601.2h, 700.2f). This applies to Fatal Lore just as it does to Abrade or Jund Charm.
Also, in the case of Game Plan or Fatal Lore, if you find yourself unable to pay the spell's cost (e.g., even assisted by another player in the case of Game Plan [see C.R. 702.132a]), then the action of casting the spell is illegal (C.R. 601.2; see also C.R. 728.1). But if an action is not illegal, it's not reversed and you must continue with the action. In a sanctioned tournament, however, a player is "not usually allowed to take back an action that has been communicated to their opponent, either verbally or physically" (M.T.R. 4.8). An exception is if a judge decides with certainty that a player "has not gained any information since taking [an unintended] action" and decides to "allow that player to change their mind" (M.T.R. 4.8). See also this thread.
EDIT (Oct. 18): One rule was renumbered in the meantime.
EDIT (Sep. 20, 2023): Edited, including to update rule numberings.
Likewise, since targets can influence costs, like in this case, targets are chosen before costs are determined.
You can legally start to cast Fatal Lore even if you don't have the means to pay for its later cost at this time. When costs eventually have to be paid, and you can't, the spell casting is nullified and the game is rewound to right before you started casting (with some actions being excempt from being reset). You may then restart the casting, and since that would result in a loop if the opponent chooses the same mode again, you as the active player will have to abandon that course of action and do something else. If you could somehow cast the spell in your opponent's turn, you can force him to choose the targeted option, because he then has to make the differing choice to break the loop as he is then the active player.
In the case of the Assist ability, you cannot force a player to pay for your spell in this manner, and will have to back out of casting the spell if it cannot be paid for, and do something else after the attempted casting is reversed.
Former Rules Advisor
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