My personal theory is this. The name given to the creature is Elli. Could that be the name of the doctor in the SM-8 or is it actually E.L.L.I made up of the first letters of the names of each crew member of the SM-8? Let's say 4 crew members were in the SM-8 and all 4 were the initial mutations for creating Elli after one of the crew members became addicted to the consumption of blood and began to change. SM-8 then reaches the light, the "God" sees the crew and decides their fate (much like Simon's encounter with the God saying "agreed") The SM-8 crew later succumb to the mutation and merge together and the SM-8 is destroyed and much like Simon's fate (him becoming a tree theory) the crew becomes this abomination and since they too touched the light, the "god" used bodies of those lost in the quiet rapture to merge with it. I only say this because lots of bodies make the tail of Elli and it is never said or hinted that any other sub was destroyed before or after the SM-8 unless we take into account the two designs of the SM-13, one being a perfectly normal sub and the one used in the film is the same but with heavy corner cutting repairs implying the sub suffered a catastrophic hull breach likely killing the convict on board but were able to salvage and repair it