In the different voicelogs found in the Degasi seabases in one they talk about how Marguareit found a injured leviathan and brought it to the base..
In a later dialogue they argue about the posibility of another leviathan coming to help its injured friend, then a creature roars like a reaper but not quite the same and destroys the base. In the final log Paul Torgal describes the creature as larger than a cyclops and with tentacles like a kraken, then he states that Marguareit stabed the monster in the neck making it flee.
I don't think it could be a reaper just because the description Paul gives, it could be a sea dragon because this happened in the deep grand reef and could have gone through the lost river.
What do you think?
(Sorry if there's grammar errors, english isn't my first language and I'm writing on mobile)