In this clip from "Rose," Damon tells Stefan about Katherine's cryptic message. This inspires the latter to want to chat with the tomb-trapped blood sucker.
Damon and Stefan set off to find Elena on the episode "Rose." Who has kidnapped her? As this preview states: Fresh. New. Blood.
Who is Rose? Who is Elijah? What do they want with Elena? We meet fresh blood on the episode "Rose," previewed here and now.

The Vampire Diaries Season 2 Episode 8 Quotes

Rose: I know who you are. I said be quiet.
Elena: What do you want?
Rose [slap!]: I said be quiet.

That werewolf road leads straight to vampire boulevard!