Scylla is a giant squid/spider Titan.
According to Greek legend, Scylla resided in the sea and would destroy any ships that got to close to her.
Scylla was contained a Monarch containment facility in a desert nearby oil rigs in Sedona, Arizona, being monitored by Monarch and designated "Titanus Scylla". After its file is briefly examined by Mark Russell, Scylla is awakened by the calls of her new Alpha, King Ghidorah, and she destroys the oil rig she was trapped under from underground. Along with the other Titans (excluding Godzilla, Mothra, and Kong), Scylla is sent to hunt, moving as a pack with the other Titans, and is seen using her legs to destroy buildings in Phoenix.
After hearing the alpha call by the ORCA, Scylla ceases her attacks and travels to Boston. However, she is too late to aid Ghidorah and arrives in Boston alongside Behemoth, MUTO 3, Methuselah just in time to witness Ghidorah's defeat by Godzilla. After Rodan bows to Godzilla, she along with the other Titans follow suit, acknowledging him as their new King of the Monsters.
Afterwards, news reports later mention that Monarch has confirmed Scylla's Greek origin, and it can be seen that Scylla has become one of the most popular and famous Titans to humanity. According to the reports, Scylla is having a positive impact on the environment by emitting enough liquid nitrogen to slow down the melting of ice in Antarctica and stabilizing the world sea levels.