$18k -> $3k (6x) wouldn't be unprecedented. In 2011, we fell from $30 -> $2 (15x), in early 2013 from $250 -> $50 (5x), and in late 2013 from $1200 -> $200 (6x).
If we get away with a low of $9k (2x) I'd hardly even call that a crash!
Here are projected lows compared to historical crashes:
2x: $9,000
3x: $6,000
4x: $4,500
5x (early 2013): $3,600
6x (late 2013): $3,000
10x: $1,800
15x (2011): $1,200
Bitcoin is less volatile now. Unless the bull market has ended - which I do not think it has - most of these other crash comparisons are not really valid.
Right now this is a healthy correction - one many people called - that will lead to another bullish year in 2018 with BTC prices well above $50k. BCH might die however

Previous corrections would go into alts but its all dropping now. It wont get to low Ks overnight but 20k could be a top. Its x10 what it costs in electricity per BTC and most past tops were never 20x of previous ATHs.