I also built my commander out of cards I had, I originally wanted to make it a White/Black/Red, but finding a commander for that was more difficult than I had time for, so I went White/Black with Sorin, Grim Nemesis as my commander.
My deck is primarily built around making the other players regret their decisions and being able to get an unstoppable force going, as such it has a few different strategies.
First, there is Sorin's ultimate (put X 1/1 black vampires with lifelink on the field where X is the highest life total of all players). One of my friends loves building up tons of health with their "life" deck, so this is a nice counter to it if I can keep Sorin around long enough to unleash it.
Then I have a Liliana of the Dark Realms, which has an awesome ultimate (tap swamps for 4 swamp mana) which combines amazingly with From Under the Floorboards' madness cost to get a ton of 2/2 black zombies on the field.
Archangel Avacyn is just great.
Descend upon the Sinful to clear the board, hard (exile all creatures). This typically isn't the best to pair with Rise of the Dark Realms, but it does clear out everything to make a move, and if you can play Congregate with Felidar Soverign right afterwards, winning within a round or two is a pretty good bet.
Necropolic Regent helps me to swell my creatures, a ton. And when paired with Always Watching, I get some extra power and a nice addition of vigilance.
Guardian of the Gateless, paired with Holy Mantel can block any number of creatures, but also has protection from creatures, making it a pretty large wall.
And finally, I have Elixir of Immortality just in case I need a reset (gain 5 life, shuffle Elixir of Immortality and your graveyard into your library).