The power essentially says "on your turn you must do A or B." (where A = engage, and B = challenge somebody else)
Let's say I have three boxes full of pebbles, and a key.
The rules:
If a box is open at the end of a round, and I have not taken a pebble out of it that round, it will explode.
I can only close a box by removing the key and opening another box.
I can only open one box in any given round.
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So I have a Box 1 open. In this round, I can either a/ remove a pebble, or b/ close Box 1 by opening Box 2. If I do neither, Box 1 will explode.
If I choose not to remove a pebble, and open Box 2, then Box 2 is set to be open at the end of the round. Unless I remove a pebble or open Box 3, Box 2 will explode... and since I've already opened a box this round, opening Box 3 isn't an option.
I must remove a pebble from Box 2 if I don't want it to explode.
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Now I'm a Paladin with three opponents, and I used Divine Challenge on opponent 1 earlier. We'll call this use of Divine Challenge C1.
The rules for C1 say that I must either engage opponent 1 before the end of the round, or challenge another opponent... or my Divine Challenge will cease to function.
If I choose not to engage opponent 1, I must therefore challenge opponent 2. We'll call this use of Divine Challenge C2.
Now that I've used the power, the rules for C2 say that I must either engage opponent 2 before the end of the round, or challenge another opponent... or my Divine Challenge will cease to function. And I can't challenge opponent 3, because I've already used Divine Challenge this round.
Therefore if I don't engage opponent 2, my Divine Challenge will cease to function. Not because I failed the conditions of C1 - I met those by challenging another opponent. Because I failed the conditions of C2.
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Consider a hypothetical power - Minor Action. Mark the target. If you do not say the target's name before the end of the round, take 1d6 damage.
I use a minor action to mark Fred, and I use a minor action to mark Bob. If I say "Fred", I won't take the 1d6 damage from that use of the power. But if I don't say "Bob", I'll still take 1d6 damage,
even though I said "Fred", because the rules apply to the second use of the power as well.
Why do some people think that anybody who does B must also do A?
People don't.
They think that in response to the requirement of the first use of Divine Challenge, he must do A or B, and he chose one of them - B (challenge an opponent other than the target of the first use of Divine Challenge). Then, in response to the requirement of the
second use of Divine Challenge, he must do A or B. But he can't choose B (challenge an opponent other than the target of the second use of Divine Challenge) this time, because of the restriction on the power of only choosing B once per round... which means that his only option in response to the requirement of the second use of Divine Challenge is A - engage the target of the second use of Divine Challenge.
-Hyp.