Unholy Annex deals 2 damage to you each turn, but if you control demons (or changelings), then it becomes a future win condition
You can set this up in the first few turns with Queen first, Nick Wilde Sly Fox Sleuth and the Queen damage ability on turn two, into Pluto turn three to play Queen again, setting you up for something similar turn four
But the second ability is kind of an upgrade to the Abzan Charm model of drawing two cards and paying two life for two cards (but three colored mana)
The power level of the Dominus and Twilight cycles, matched with the speed boost games will get from an increased presence of poison counters, means that this set is going to have a huge impact on Commander
Counter Blitz: Boost team spirit by moving counters from creature to creature
When you have removal sitting in your command zone that hits indestructible creatures, its hard for your opponents to feel safe committing anything to the board