What is a "Phantom Clock" RTC, like the Maxim DS1315 and how is it different to "regular" RTCs?
The normal RTC chips have fixed addressing and are normally separate structures within a solution. 
A Phantom RTC, works in the same address space as an existing SRAM chip, and has an on board control that allows either normal SRAM R/W or the special serial location R/W required to get/set the clock data.

You would only use a phantom RTC device where you had severe MCU configuration constraints.