Time stop and ready action

Question

A spellcaster casts a Time stop that lasts 5 rounds (he knows how many rounds of time stop he has) and proceeds to buff himself for 4 rounds. He then does the "ready action" standard action on the 5th round stating that the triggering event is "when the apparent time of my time stop spell expires I cast [name of a targeted spell e.g. finger of death] on [name of the target]. Is this allowed by RAW? Thanks.

Answer 1

RAW is ambiguous, your DM must resolve this

The Readying an Action rule says:

Readying an Action

You can ready a standard action, a move action, or a free action. To do so, specify the action you will take and the conditions under which you will take it. Then, any time before your next action, you may take the readied action in response to that condition. The action occurs just before the action that triggers it. If the triggered action is part of another character’s activities, you interrupt the other character. Assuming he is still capable of doing so, he continues his actions once you complete your readied action. Your initiative result changes.

This rule first says that you specify "conditions" that trigger the readied action, and you can take the readied action in response to that condition (singular). Time stop ending would be a condition, and even one that you can observe, because time stop says

This spell seems to make time cease to flow for everyone but you. In fact, you speed up so greatly that all other creatures seem frozen

You therefore should be able to see when the other creatures stop to seem fozen, and then take your readied action to cast finger of death. Even though time stop stipulates that "While the time stop is in effect, other creatures are invulnerable to your attacks and spells; you cannot target such creatures with any attack or spell", you are not doing that. The ready action lets you prepare to take an action later, after your turn is over, so by the time you take that action, time stop is not in effect any more.

However, there is one issue. The ready action rule continues to say that you can take your readied action just before the action that triggered it. This implies that the condition that triggers your reaction must be some kind of action. One example given in the full text of the rule is "if they start casting a spell", which would be an action. And time stop ending is not any kind of action, so it would not serve as a condition in this context.

I do think it is ambigous how this is intended to work. Purely as written, I think the trigger needs to be an action, but this can just be sloppyness in formulation, like that the rule refers to both "conditions" and "the condition" in the space of the same paragraph. The large majority of conditions typically are some action of an opponent that you want to mess with, like the opponent attacking, moving or trying to cast a spell. The sentence explains which of two actions is processed first, if you ready in reaction to an action -- yours is, so maybe it should be seen in that context, not as a general statement that the condition must be an action. Conditions in general would include time stop ending.

You can argue either way, so I think it is best that the DM makes as call here, ideally in advance, and that is how you then play it.