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Spock Prime contacts his "old friend" after the Enterprise's first five year mission is complete, to inform him of two things. One, he doesn't have long to live. Two, he's going to live long enough to go through pon farr one last time. Since the universe has provided him with such an opportunity... (As for warnings: End of life issues, but Spock Prime has lived a long full life, and he's at peace. Take that as you will.)