Hey all, I'm a rhythm guitarist but I'm not exactly creative. I want to know where to start to write riffs that sound good for this genre of music.
Spend more time analysing metalcore songs; find out what makes them work. From there writing your own is relatively easy.
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I'll explain in a few easy steps :P

1) Detune your guitar so that your 5th and 6th strings are a 5th apart.

2) Put all the strings lower still. 1 or 1 1/2 steps down usually works.

3) remember to always use the open lowest string as a pedal note. This will add technicality to your riffs, due to the octave leaps that you'll be making.

4) Use the notes in the natural minor scale that is relative to the tuning of your lowest string. For example, if your 6th string is tuned to B, then use the notes B, C#, D, E, F#, G and A in-between your pedalled open B.

5) If you're feeling adventurous, then feel free to change the chord that you play over. Good ones are III, V and VI. You can find these on your guitar by looking at the 3rd, 7th and 8th frets on your 6th sting respectively.

6) After you have this, then you can add in your generic breakdown and clean chorus

Some great examples of metal core riffs are:
-Avenged Sevenfold; Unholy Confessions
-Asking Alexandria; The Last Episode

I hope that you found this useful and good luck in writing the next big hit that will get all the 14 year old girls dancing

PS. I know that it's stereotypical of the genre. It's a joke based on the fact that a lot of bands do this xD There are some decent metalcore bands out there! So yeah, in all seriousness, listen to your favourite riffs, learn them and then mix and match certain parts to create something similar until you can mould your own personal style
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