The creature sat upright and tugged at the shackles binding his ankles, Frankenstein uttered a piercing scream.

Question

How do you rewite a fused sentence

Fused sentences and comma splices are just variations of the same thing -- run-on sentences. Just be sure you put a period at the end of each sentence (or ? or ! if needed) and a capital at the beginning!

http://grammar.ccc.commnet.edu/grammar/runons.htm

That sounds like one sentence so how would I add a period

It's two sentences. Did you read the information at the link I sent?

The creature sat upright and tugged at the shackles binding his ankles

Frankenstein uttered a piercing scream

How will you make it clear that these are TWO sentences??

??

by adding a period behind ankles

Yes, that's the basic way.

You could also write When the creature sat upright and tugged at the shackles binding his ankles, Frankenstein uttered a piercing scream.

By adding "when" and making the first clause subordinate, the relationship between the two sentences is clearer and it doesn't sound so choppy.

=)