Needs More Examples, Needs A Better Description.

Sometimes you need a complex ritual to free the Sealed Evil In A Can...but sometimes you just need a little blood. Not to say this can't be done as part of a ritual, but in many cases it's by accident, where splattering some blood in an innopurtune time frees, awakens or revives a sleeping evil.

Examples:

• Dracula would always be raised this way in the Hammer films.
• In Hellraiser, the husband bleeding on the floor is what allows his evil undead brother to regenerate himself.
• In the manwha Priest, a train secretly delivers a cargo of zombies. They were supposed to stay dormant for the trip, but then a train robbery splatters blood on their door...