Guru Nanak Sahib’s critical contribution in the mystical domain is the process of transmission of spiritual power (Nam, Hukam or Divine Shabd or the Word or the JOT) to another human being, Guru Angad Dev Ji, his successor and to make him akin to his predecessor -- with mandate to initiate other souls with the same technique (Jukti). The string of succession is "historically" understood up to the tenth Nanak—Guru Gobind Singh Ji. Thus each Guru, despite living in different physical formats, carried the spiritual light or the same “Jot of Roohaniyat” for the benefit of other human beings of that time.
The word “Waheguru” that describes the glory and greatness of the Guru was first uttered by Guru Angad Dev Ji, the second Nanak. This is in accordance with the legends that are associated with spiritual ascensions of the Perfect masters. There could always be a sense of disbelief to the mystic narratives but there is no smoke without a fire.
In the cosmic hierarchy of the spiritual Masters- Pind, Anda, Brahmanda (the region with least degree of mind and matter) - soul has to finally ascend to the region of pure spirit- Satlok or Sat-desh for salvation and redemption. The intervening region between Barhamanda and Sat-desh is full of pitched darkness –also called as Timir Khand, which is an expanse of potent illusion-where darkness appears to be expanding, — and which is understood to exist in an astral form after Daswan Dawar( the tenth door from where the soul goes beyond Brahmanda). Conceptually this darkness is literal and not analogical or metaphorical and not to be confused with darkness of ignorance. The soul when devoid of mind and matter has its own powerful light-- said to be equivalent to 12 suns. But it gets lost or is engulfed in abyss of intensifying blackness, unless further progression is enabled by potent brilliance of light of the Guru in the Timir Khand.
When Guru Angad Dev Ji experienced the inward ascension of his soul in Timir Khand, full of pitched darkness, he was simultaneously welcomed by the potent brilliance of the light of his Master, Guru Nanak Dev Ji to cross that region of absolute darkness for proceeding to Satlok. Guru Angad Dev Ji, then in a state of astonishment and bliss mentioned/uttered the sacred word of “Waheguru”—the Guru is Unique, Sublime the Greatest-- as a humble obeisance to his Master.
He described his experience as “जे सउ चंदा उगवहि सूरज चड़हि हजार ॥ एते चानण होदिआं गुर बिनु घोर अंधार (Page 463 SGGS). If a hundred moons were to rise, and a thousand suns appeared, even with such light, there would still be pitch darkness without the Guru”. His words are full of mystical and esoteric connotations. Logically the succession of the subsequent Gurus would have happened likewise.