Reducing a creatures toughness to 0 means its put into the graveyard the next time state-based actions are checked, usually before a player gains priority
For example, he deals combat damage to any creatures he's blocking, but he doesn't deal combat damage to any unblocked creatures that are attacking him.
Descend 4 Didact Echo has flying as long as there are four or more permanent cards in your graveyard
Theres still room for off-color or non-mana plot costs in future sets, but in its first set, we also already see a couple of cards that trigger some ability when plotted
Cards that Create Toxic Tokens These are cards that don't actually have toxic themselves, but creature tokens that do
I wrote a whole lot about Land Tax in my Top 50 White Cards article, so I'm not going to go on and on about it here