Cheapest priced copy (Commander 2013): $1.29 at Card Kingdom, near mint, their price list as of .
Serene Master 
Creature — Human Monk · 0/2 · Rare · Commander 2013 · #20
Whenever this creature blocks, exchange its power and the power of target creature it's blocking until end of combat.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $1.29 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2013 | 20 | Rare | $1.29 | — |
4 rulings from Wizards of the Coast
- 17 October 2013The exchange is made only if both Serene Master and the target creature are on the battlefield when the ability resolves.
- 17 October 2013Any 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.
- 17 October 2013If 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.)
- 17 October 2013When 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.