EMB505 pilot reported taking evasive action to avoid a collision with a flight of four aircraft descending through their altitude.

Narrative

Aircraft X Part 135 with 5 passengers departed ZZZ for ZZZ1; ZZZZZ departure ZZZ1 transition. ATC assigned 11000 feet. At XA35; while on ZZZZZ1 to ZZZZZ2 leg almost to ZZZZZ2; crew observed one aircraft on TCAS display descending from 1000 feet above our assigned altitude. SIC made visual of several skydive type multi-engine aircraft including a Twin Otter in close formation. SIC called out aircraft as RA to climb was activated and immediately followed by an RA to descend as 3 of the 4 aircraft were co-altitude and the trail aircraft slightly higher. The PIC reacted to the climb resolution and descend resolution with a turn to the right. The 4 aircraft passed within 150 to 200 feet and without maneuvering; collision was imminent. Our altitude deviation was 11100 down to 10500 feet with recovery back to 11000 within a minute of clearing resolution. ATC was notified of the collision avoidance as it happened. ATC was on XXX.X and never warned us of the traffic conflict. ATC stated to the jump aircraft they were not supposed to descend on the departure airway and nothing else was stated. Flight continued to ZZZ1 without further event.Suggestion: ATC should have been more vigilant and warned us of the potential conflict. The 4 skydive aircraft were not situationally aware and distracted with their formation flight. ATC never issued any corrective action to all involved.