It doubles any source of damage, not just creature attacks, red sources, or burn spells
Many decks can do a lot with tons of tokens dumped into play, especially since this is a 6-for-1 or some nonsense
Five toughness, immune to sorcery speed things should you like and being black heavily limit what actually deals with him and unlike other medium sized no frills fatties you have to kill him, you can't just stale mate and wait for something else
If luck is on your side, you can fully power up Lapras ex or Articuno ex in one turn
The only decks that existed for some time were ramp decks featuring Field of the Dead, and 22 Small Probe, Big Problem (Gitaxian Probe) Sometimes, it only takes a small and unlikely spell like Gitaxian Probe to cause some serious damage
which led to a whole ton of ruling headaches because it 'puts cards face-down'