In September of 1993, on a dusty table in a dusty basement, some poor sap waited until his opponent’s turn to Lightning Bolt a Grizzly Bears. His opponent cast Giant Growth attacked for 5, and thus began a long and painful history of Magic players casting their removal spells at the wrong time.

There’s a seemingly simple interaction that comes up incredibly often in Standard, which is a Courser of Kruphix being targeted by a removal spell. The interaction is seemingly simple, but below the surface is actually quite complex, involving challenging concepts like information management and long-term planning. For such a common interaction, the frequency with which it’s done correctly is astonishingly low. Even I, who have spent many sleepless hours thinking about (and now writing about) this very question, bungle it far too often.

But let’s start from the beginning. A newer player probably wouldn’t think twice before casting Hero’s Downfall on his opponent’s Courser of Kruphix right away, during his own main phase. A slightly more sophisticated player would wait until the opponent’s draw step to do it. In this way, she would get to see the new top card of her opponent’s library, but the opponent would not get to play a free land nor gain life off of the Courser. The problem is that most players seem content to stop thinking about the question at this point.

To always kill the Courser during the opponent’s draw step is worse—far worse—than to always kill it in your own mainphase. The reason is that in most games of Standard, information about the top of a player’s library is far more valuable to that player than it is to you.

Consider that decks with Courser of Kruphix can contain any combination of fetchlands, scry lands, Satyr Wayfinders, card drawing, other cards that shuffle the library, other effects that scry, and other effects that mill cards. These things allow the player with the Courser to manage the top of his or her library while the opponent can typically do very little with the information. When you’re considering allowing your opponent to reveal an extra card from the top of his or her library, you’re typically weighing your own curiosity against an actual, tangible advantage for your opponent.

Example:

Your opponent draws a card, and reveals their next card—Llanowar Wastes—during their draw step, you cast Hero’s Downfall on the Courser. Your opponent then casts Satyr Wayfinder, mills cards from the top of his library, finds a land, and sets up a fresh, random draw step for next turn.

Alternatively, he reveals his next card and it’s Hornet Queen. Now he declines to cast the Satyr Wayfinder, ensuring a powerful draw step for his next turn and saving the Wayfinder for later.

If I had to offer a general rule, I’d say to always kill the Courser in your own main phase. However, there are two circumstances where it can be in your best interest to wait until your opponent’s draw step.

First is when you have complete information and know that your opponent cannot manipulate the top of his or her library. Remember to check for fetchlands in play! Even once this is established, you must still decide that the information will affect your own decisions more than your opponent’s. If she reveals the new top card of her library and it’s Siege Rhino, now she can make what might have otherwise been an unfavorable attack because it puts you to 3 life. In that case, giving her the free information might have cost you the game!

The second circumstance is when you’re legitimately undecided about whether or not you want to kill the Courser, and the top card of your opponent’s library is going to help you make the decision. For example, say the game is completely even and you’re at 17 life, unsure whether you want to kill your opponent’s Siege Rhino or her Courser of Kruphix. It could be that the presence of a land on top of her library will make you decide to kill the Courser, whereas the presence of a second Siege Rhino will make you decide to kill off the more threatening attacker.

As with all things, the key is simply to be thoughtful and consider the subtleties of the game in front of you. More often than not, the information will be more valuable to your opponent than it will be to you. But use your judgment as to when the reverse might be true! Just make sure to never be careless, even with a decision that may at first seem trivial.