A temporary phase of prohibition and fierce persecution of Buddhism and its monastic community under the Tibetan king Langdarma - until he was assassinated for that very reason by the deeply compassionate Lhalung Pelgyi Senge (one of Padmasambhava's 25 early initiates) who did not want this king to suffer the eternal consequences of heaping too much bad karma upon himself.
As an effect of this, Tibet plunged into a kind of dark age dominated by political struggles, internal wars and territorial losses to China; from which emerged a less centralized country with separate dynasties in Central and Western Tibet.
While monasteries were closed and monks had to leave the country or go underground, the teachings were mainly preserved by the non-monastic and independent Tantrics known as ngagpa (male) and ngakmo (female.)