Cleary Gottlieb Steen & Hamilton and Simpson Thacher & Bartlett are advising on a multibillion-dollar deal that will see Japanese and South Korean tech giants SoftBank and Naver Corp. embark on a joint venture.

SoftBank said in a statement that its subsidiary, Yahoo Japan, which changed its name last night to Z Holdings Corp., will merge with Naver-owned Line Corp., creating an internet giant. The deal will create a $30 billion business, according to Reuters.

Line is one of the dominant chat apps in Japan, claiming to host over 80 million active monthly users. The deal will also involve a buyout of Line's minority shareholders.

The two firms will enter a 50-50 venture that will own Z Holdings, which in turn owns both Yahoo Japan and Line.

Simpson Thacher is advising SoftBank, alongside Nagashima Ohno & Tsunematsu, one of the four largest law firms in Japan.

Cleary Gottlieb is advising Naver, with a team comprising U.S. partner Paul Shim and South Korean partners Jinduk Han and Sang Jin Han. Another of Japan's largest four law firms, Nishimura & Asahi, and South Korean law firm Kim & Chang, are also advising Naver.

Earlier in the year, Sullivan & Cromwell and Davis Polk & Wardwell advised SoftBank on an initial public offering for the conglomerate's domestic telecoms business on the Tokyo Stock Exchange, which at the time aimed to raise $21 billion.