In October 2014, we were all caught off-guard by the rumours of MBLAQ’s Thunder (Cheondung) leaving the K-Pop idol boy group. It was later confirmed in December 2014 that, yes, he was indeed calling it quits, alongside another member Lee Joon following the expiration of their contracts with label J. Tune Camp.
Fans were left wondering about their future plans as it was stated that both Lee Joon and Thunder will be moving forward with their individual activities. At that time, it was yet to be known if they will sign a contract with J. Tune Camp and continue to be managed by the agency.
Source: ohthunder.wordpress.com
Just last month, Sports Donga wrote that Thunder officially signed with APOP Entertainment, a sister music label under MYSTIC Entertainment.
Upon the official announcement, a representative of the agency said:
Thunder is an artist that can be distinguished in many different fields with an endless amount of talent and potential. We plan on supporting Thunder in not only expanding his singing career, but also aiding his growth in both his acting skills and variety skills. We also plan on wholeheartedly aiding Thunder’s acting through MYSTIC Entertainment’s sister acting label, Family Actors.
It was revealed that Thunder came to sign with both MYSTIC Entertainment and APOP Entertainment through Cho Young Chul, a music producer for the 2 sister agencies.
Now, it has also been confirmed that Thunder will be dropping his stage name as he pursues his acting career. This means that from this point onwards, he will no longer go by the name “Thunder”. Instead, his future activities will include his birth name, Park Sang Hyun.
Ex-MBLAQ member “Thunder” aka Park Sang Hyun with his sister, Sandara Park of 2NE1
The first of his future projects with his birth name will materialise this April in the upcoming MBC weekend drama, “Let The Girl Cry” where he will play Hyun Seo, the nephew of Jin Woo (played by Song Chang Ui).
Well, can’t say that we’re not excited about watching Park Sang Hyun in action, real name and all 
Sources: Sports Donga (1), Sports Donga (2), Koreaboo.