Kundalini shakti is Amrita (elixir of life) that lay dormant in the base of spine. When seekers, travelers of spiritual path indulging in spirituality activated it to reach crown chakra, also called Ajna chakra the seeker reached state of Shiva!
Shiva is that attribute of God Almighty that is considered most powerful. After reaching state of enlightenment the spiritual traveler gains omniscience, becomes knower of all forever.
In the neck of Lord Shiva we find lots of coiled serpents. These coiled serpents denote a full awakened kundalini. The dark bluish body represents an enlightened one drinking the Vish (poison) generated during Samudra Manthan.
The Vish (poison) can be retained in the neck or left loose. If retained in the neck it represents the dark black ring in the neck of Shiva and also Lord Krishna. If the Vish (poison) spreads to entire body, it is represented in the dark blue body belonging to both Shiva and Lord Krishna.
Every enlightened one gets a dose of Vish (poison) that could either be retained in the neck or let loose to spread in the body. Sri Ramakrishna Paramhansa retained the Vish (poison) in the neck which became the cause of throat cancer by which he finally died.
Spiritually one becomes a Shiva by awakening kundalini energy to its full potential. Every single enlightened soul reaches state of Shiva at the end of spiritual journey.
Moment an enlightened one (Shiva) left mortal frame one becomes Vishnu, the liberated soul atman. Lord Vishnu is always shown lying on Shesha (multi-hooded king of serpents) with Lakshmi as consort. Shesha represents an awaken kundalini, the ocean entire cosmos and Vishnu a liberated soul atman that could venture to any part of cosmos at will.