All of these were printed early on in Magic's lifetime - in Alpha, Beta, or Unlimited - before balance had been fully figured out
While the front side is good enough to see play on its own, the fact that it only has four defense counters makes it relatively easy to flip (a single Sheoldred, the Apocalypse attack gets the job done), and the backside offers a very powerful creature that can defeat a battle or kill a planeswalker each turn while potentially generating card advantage as well
The supply increase wasnt completely suddenit was a shift over time that just happened to hit equilibrium around the start of M13
Reanimate + Animate Dead Black is famous for the cheap reanimation effects it provides, and those are among the best
You must sacrifice exactly one creature to cast this spell
It's also a strong copy effect in control brews that love to get card flow into the creature base with this since you can copy your own Consecrated Sphinx to draw tons of cards, Thassa's Oracle to win or your foe's Karmic Guide to bring back something key and then draw when it swings