᯽The reprehensible curse᯽

Nandi(Lord Shiva's Vahan) cursed Ravan, saying that monkeys would be the cause of his demise.

Once Ravan was flying back to Lanka in the Pushpak Vimana after defeating Kuber (the Lord of Wealth) and looting Alakapuri. On his way back, he noticed a beautiful mountain, Mount Kailasha, but was unable to fly over it.

Ravan then attempted to enter Kailash, but Nandi stopped him.
He inquired as to the cause of his inability to pass.
Nandi told him that his Shiva and Parvati were enjoying dalliance on the mountain and nobody was allowed to pass. Upon hearing this, Ravan referred to Nandi as a Vanar (monkey) and attempted to enter and uproot Kailash.
Nandi cursed him saying that Vanars would destroy him.
The curse materialised when the Vanar Sena defeated Ravan later in the Ramayan.
Ravan kept shaking the mountain.
A terrified Parvati embraced Shiva.
Mahadev being ubiquitous recognised the cause and positioned the mountain by pressing his big toe.
Ravan was crushed beneath the mountain.
He sobbed in agony.
For the next thousand years he performed extreme penances to please Shiva.
He offered up his head and sang praise songs, including the Ravanrachit Shiv Tandav Stotram.