A third-party complaint — the procedure known as impleader — lets a defendant bring a new party into an existing lawsuit, arguing that party is liable for some or all of the plaintiff's claim against them. It's a tool for shifting or sharing liability rather than fighting the whole case alone.
The claim has to be derivative
Impleader isn't for asserting an unrelated claim against a third party — it specifically requires that the third party's liability derives from the original claim, such as an indemnification or contribution obligation.
Timing rules are stricter than for the original complaint
Most courts allow a third-party complaint to be filed without permission within a set window after the defendant's answer, but require court approval if filed later — missing that window adds an extra procedural hurdle.
It has to stand on its own as a complaint
A third-party complaint needs the same elements as any complaint — properly pled claims, jurisdiction, and a clear statement of the third party's alleged liability — not just a reference back to the original lawsuit.
Consider the strategic tradeoffs before filing
Impleader can complicate a case with additional parties and discovery, so it's worth weighing whether a separate action or a settlement demand against the third party might resolve the exposure more efficiently before adding them directly to the litigation.