Doomed Traveler - Triggers! Plus, a free block that makes a 1/1 flyer is insane.
Dragon Egg - A relatively cheap blocker that creates a pumpable flier when it dies? VALUETOWN!
Kathari Bomber - Need a flier to get over some ground dwellers? BOOM! 2/2 flier. Need triggers? BOOM! Double triggers when bomber deals combat damage to a player. It has Unearth as well for the late game, which is very much appreciated in this deck.
Mogg War Marshal - Marshal + Reap + Undying = for two cards, five 1/1s (TRIGGERS!), and a free 2/2 blocker. Lord still my beating heart!
Ponyback Brigade - Staple of the deck that becomes a nice blocker with Undying Evil that creates 4 mother-freakin (excuse my French) TRIGGERS!
Soul Warden - Put those triggers to use!
Soul's Attendant - See above. The split is to protect against cards like Echoing Decay.
Impact Tremors - Kill 'em 'till they're dead! Need I say more?
Altar's Reap - This card was meant for this deck. I mean, block a threat with a Doomed Traveller or something, then Altar's Reap in response (yeah it's instant speed, sucka), and there ya go. BOOMSHAKALAKA!
Lightning Bolt - This deck has Mountains.
Raise the Alarm - Instant speed double triggers. Don't get much better. They can block too.
Undying Evil - Extra triggers. This also can make a free blocker that gets bigger, or turn a sacrifice into an even bigger advantage. It's also the other half of the key combo with Ponyback Brigade. (See COMBOS)
LANDS:I run a 5-5-3 ratio of Mountains, Plains, and Swamps. I also run 3 Holdout Settlement to support my three colors and help with splashing. I also run 4 Evolving Wilds to help with mana fixing. This makes for a total of 20 lands. This amount is fine for my relatively low converted mana cost, but you may choose to add more or fewer.