Serene Master

• WWizards of the Coast2013-10-17
  Official Ruling
  The exchange is made only if both Serene Master and the target creature are on the battlefield when the ability resolves.
• WWizards of the Coast2013-10-17
  Official Ruling
  Any power-modifying effects, counters, Auras, or Equipment will apply to the creatures’ new powers. For example, say Serene Master is enchanted with Lightning Talons, which gives it +3/+0, and it blocks a 5/5 creature. After the exchange, Serene Master would be an 8/2 creature (its power became 5, which was then modified by Lightning Talons), and the other creature would be 3/5.
• WWizards of the Coast2013-10-17
  Official Ruling
  If Serene Master blocks multiple creatures, its ability will trigger only once, and it will exchange power with only the target creature. (Note that Serene Master can’t block multiple creatures normally.)
• WWizards of the Coast2013-10-17
  Official Ruling
  When Serene Master’s ability resolves, its power becomes equal to the former power of the target creature. At the same time, that creature’s power will become equal to Serene Master’s former power.

Rulings via Scryfall