Here's an attempt to summarize the process as an ability. I think we should change the 9 rounds and 3 rounds to variable amounts, possibly 3d3 (or 1d8+1) and 1d3 rounds?
Making it variable suits me. How about 2d4+1 (3-9) rounds normally, 1d3 (1-3) rounds if slain by fire?
Rise From the Ashes (Su): When a phoenix spider is slain (reduced to -10 hit points or below), its body is immolated, leaving nothing but ash. Nine rounds later, the phoenix spider reforms from the ashes, larger and stronger . If the "killing blow" contains fire damage, the phoenix spider instead reforms 3 rounds later in the same location where it perished.
What happens if the spider is knocked unconscious (0 hp) or dying (-1 to -9 hp) and gets stabilized? Indeed, a party that is familiar with the monster might try to stabilize its wounds to prevent it reforming.
Maybe we should make that "reduced to 0 hit points or below" rather than -10?
I'd also move the "in the same location" bit into the first sentence on it reforming.
It also occurs to me we want to say the spider reforms at its full hit points. I think an easy way of doing it is comparing it to a
true resurrection.
When the phoenix spider reforms, use the stat block for the next larger size. Thus, a Small phoenix spider rises as a Medium phoenix spider, a Medium as a Large, and a Large as a Huge. A Huge phoenix spider reduced to -10 hit points begins the procees in reverse, arising as a Large phoenix spider. This reformation continues until the spider is slain in Small size. At this point, it is permanently destroyed.
At any point in the process, should an attack that deals cold damage reduce the phoenix spider to less than -10 hit points, the spider is permanently destroyed.
This second and third paragraph look fine to me, except for the -10 hp problem I mentioned earlier.
If I apply all these suggested modifications we end up with something like:
Rise From the Ashes (Su): When a phoenix spider is reduced to 0 hit points or below its body is immolated, leaving nothing but ash.
X rounds later, the phoenix spider reforms from its ashes at the location it perished in a larger and stronger form. If the "killing blow" contains fire damage, the phoenix spider instead reforms
Y rounds later.
A phoenix spider reforms from its ashes at its full hit points, vigor and health, as if it was the recipient of a
true resurrection spell.
When the phoenix spider reforms, use the stat block for the next larger size. Thus, a Small phoenix spider rises as a Medium phoenix spider, a Medium as a Large, and a Large as a Huge. A Huge phoenix spider reduced to 0 or fewer hit points begins the process in reverse, arising as a Large phoenix spider. This reformation continues until the spider is slain (reduced to less than -10 hit points) in Small size. At this point, it is permanently destroyed.
