The safest, highest-quality method for a true digital library is to buy the official DVD box set (check eBay or Korean bookstores like Aladin) and rip it. Use software like MakeMKV or HandBrake to convert the VOB files into MP4 or MKV. This is 100% legal for personal backup.
If you're interested in downloading a legitimate copy of the series, here are some technical details: download emperor of the sea season 1
If you decide to legally, plan your storage accordingly. The safest, highest-quality method for a true digital