Leechers are newbloods who can siphon life from plants, animals and humans. It is believed that this ability could be used in reverse to push life into something.
"Leechers can drain life from organic matter and organisms, ranging from plants to animals to humans. Further study is required for such an interesting and dangerous ability. Researchers hypothesize that this ability can be used in reverse, to push life into something, or that an opposing ability exists among the newblood population but has yet to be discovered." -Victoria Aveyard "War Storm" "On Silverblooded Abilities"