I've read Kashi is absorbed into trishul at the time of Dissolution. But Trishul too has to be absorbed. Can anyone give references about the Trishul? As it has to be absorbed, So Kashi ultimately absorbed into Brahm? Please take into account both subtle & gross meaning of Trishul.
As described in answer here Kashi only survives Kalpantaka Pralaya:
ब्रह्मणश्च दिने सा हि न विनश्यति निश्चितम्।
तदा शिवस्त्रिशूलेन दधाति मुनयश्च ताम्।। २३ ।।
Even after the completion of a divine day of Brahmā, the city of Kāšî does not get destroyed. O Sages, at that point of time, Siva holds it over the tip of his trident.
पुनश्च ब्रह्मणा सृष्टौ कृतायां स्थाप्यते द्विजाः ।
कर्मणां कर्षणाचैव काशीति परिपठ्यते॥ २४॥
O Brähmanas, then Brahmā again recreates the universe, then he again establishes Kāśī. Because of the bondages of the karmas and their attraction thereto, it is called Kāši.
However In the Mahapralaya neither Kashi nor Trishula remain because in the state of MahaPralaya the universe is in Nãsadiya state (Neither existing nor non-existing). Name-form aren't differentiated and even space is not demarcated. Shiva Purana Rudra Samhita Shristi Khanda Chapter 6 describes state of MahaPralaya in the light of Nãsadiya Sukta of RigVeda. It states:
महाप्रलयकाले च नष्ट स्थावरजङ्गमे।
अप्रधानं वियच्छून्यमन्यतेजोविवर्जितम्॥ ५॥
अव्यक्तगन्धरूपं च रसत्यक्तमदिङ्मुखम्।। ६ ।।
इत्थ सत्यन्धतमसे सूचीभेछे निरन्तरे।
तत्सद्ब्रहति यच्छुत्वा सदेकं प्रतिपद्यते॥७॥
इतीदृशं सदा नासीद्यतत्सदसदात्मकम्।
योगिनोऽन्तर्हिताकाशे यत्पश्यन्ति निरन्तरम्॥८॥
अमनोगोचरं वाचां विषयं न कदाचन:।
अनामरूपवर्णं च न च स्थूलं न यत्कृशम्॥९॥
न यत्रापचयः कश्चित्तथा नोपचयोऽपि च॥ १०॥
अविधत्ते सचकितं यदस्तीति श्रुतिः पुनः।
सत्यं ज्ञानमनन्तं च परानन्दम्परम्महः॥ ११॥
निर्गुणं योगिगम्यञ्च सर्वव्याप्येककारकम्॥ १२॥
निर्विकल्पं निरारम्भ निर्मायं निरुपद्रपवम्।
अद्वितीयमनाद्यन्तमविकाशं चिदात्मकम्। १३॥
यस्येत्थ संविकल्पं सञ्ज्ञासञ्ज्ञोक्तित: स्म वै।
कियता चैव कालेन द्वितीयेच्छाऽभवत्किल॥१४॥
अमूर्तेन स्वमूर्तिश्च तेनाकल्पि स्वलीलया।
सर्वैश्वर्यगुणोपेता सर्वज्ञानमयी शुभा॥ १५॥
At the time of Great Dissolution when all the mobile and immobile objects of the world are dissolved everything gets enveloped in darkness, without the sun, planets and stars. There is no moon. The day and the night are not demarcated. There is no fire, no wind, no earth and no water. There is one unmanifest primordial being. The whole firmament is one complete void, devoid of all Tejas elements. There is no Dharma or Adharma, no sound, no touch. Smell and colour are not manifest. There is no taste. The face of the quarters is not demarcated. Thus when there is pitch darkness that cannot be pierced with a needle and what is mentioned in the Vedas as “The Existent and the Brahman" is alone present. When the present visible world is not in existence, the Sat Brahman alone is present which Yogins observe per- petually in the inner Soul, the inner Firmament. It is incomprehensible to the mind. It cannot at all be expressed by words. It has neither name nor colour. It is neither thick nor thin. It is neither short nor long. it is neither light nor heavy. There is neither increase nor decrease in it. The Veda says that it envelops whatever is in a surprising way. It is the splendour, the truth, the knowledge, the eternal and the great Bliss. It is immeasurable, propless, changeless, formless, attributeless, perceptible to the Yogins, all-pervasive and the sole cause of the universe. It is free from alternatives. It has no beginning. it is free from illusion and its harassment. It has no second. It has neither beginning nor end. It has no development. It is in the form of pure knowledge. People have doubts about giving it a name. That Being then after sometime, it is said wished for a second. The Being, having no form of its own, wished to create, in the course of its own sport, an auspicious form of its own endowed with all power, qualities and knowledge.
Then again in next creation (ie. After Mahapralaya), Shiva and Shakti again recreate Kashi:
युगपच्च तया शक्त्या साकं कालस्वरूपिणा।
शिवलोकाभिधं क्षेत्रं निर्मितं तेन ब्रह्मणा॥ २७॥
तदेव काशिकेत्येतत् प्रोच्यते क्षेत्रमुत्तमम्।
परं निर्वाणसङ्खयानं सर्वोपरि विराजितम्॥२८॥
The same Parabrahman, in the form of Kāla, in company of that Śakti, collectively built the Sivaloka. The best of that place is known as Kasi. The place bestows salvation, therefore it is considered as Superior to all.
So, it's clear that during MahaPralaya, Shivas Trishula also get absorbed as there are neither name-forms nor space.