Woe betide somebody
Definition 1 of 1
Pro Tip 1/3Archaic Warning
The phrase is old-fashioned and not commonly used in modern, casual conversation. 
In tales, a witch might say, 'Woe betide the thief who steals my potion.'
Pro Tip 2/3Serious Tone
Typically conveys a serious or ominous warning of consequences. 
Woe betide anyone who crosses the queen's orders.
Pro Tip 3/3Conditional Threat
The phrase is often used in conditional sentences to introduce the negative outcome. 
Woe betide the team if they don't win the match; the fans will be furious.