Fearless Units
Any units which are described as being Fearless never have to fall back and are assumed to automatically pass any Morale checks they are required to take, even against attacks which normally force their target to fall back with no test taken.
Errata — Chapter Approved: Codex: Chaos Space Marines Q&A (v3.0): This entry's wording should be standard with how Fearless works everywhere else in Warhammer 40,000: Fearless also confers immunity to Pinning tests, not just Morale checks.