Everything I learned about chromaticism I learned from Slayer, specifically on songs written by Hanneman like "South of Heaven", "Dead Skin Mask", "At Dawn They Sleep", and "Raining Blood".
I first encountered Slayer in 1988 when South of Heaven was brand new. I'm pretty sure I had heard the name before, but know I had not heard the music until a distant cousin played it for me the first time while he was staying with my great aunt who lived near my grandparents.
Until then, I couldn't conceive of anything heavier than Metallica or Megadeth, two of my favorites at the time. Boy, was I wrong.
While the speed and aggression were awe-inspiring, it was the very different nature of their riffs that really drove it home. The Satanic imagery was even more of a bold announcement that this was something *really* different.
Prior to Slayer, the metal bands who had a bit of Satanic imagery like Venom, Bathory or Celtic Frost had something about them that let in just a seed of doubt that they were just kidding (e.g., Venom's more tongue in cheek songs like "Teacher's Pet" or "Red Light Fever").
Slayer was the first band where the visual, musical, and lyrical package was so focused on the darker side of things that you couldn't help but think, "These guys are for real!". Of course they ultimately weren't, in the sense of being actual Satanists or serial killers, but they had an uncompromising vision that left no cracks in the image they portrayed.
This single-minded vision, blending the musical, visual, and lyrical influenced both black and death metal bands immensely.
They also represented the final break in metal from the blues and traditional rock structures and tonalities. Even though the fast majority of their songs were based around E minor (technically Eb minor, since they tuned down a half step), they very rarely fit into any definite tonal or modal structure from Hell Awaits onward. Any note was fair game. Musically, they went from sounding like Judas Priest on caffeine, to sounding like Schoenberg on methamphetamines.
Not to diminish Kerry King's contributions at all, but where King stretched the boundaries of what was possible in raw speed in riffing, Hanneman took it to new (an)harmonic heights. In Hanneman's riffs, tritones almost sounded consonant; when any note is fair game, none of the traditional note relationships really apply.
All of this made a big impression on a 14 year old who had only recently begun playing guitar, and absorbed quite a lot of this total disregard for the rules. (It also made for a nice contrast to the more melodic bands I listen to now as then like Iron Maiden).
I haven't paid much attention to Slayer since the mid 90s. At this point, they are kind of like that old friend you rarely speak to, but every once in a while you want to hear their voice. And then, every once in a blue moon, you can't get enough of them for a couple of weeks.
But they are still part of who you are, and all it takes is a couple of notes out of the blue to remind you of them, and to miss them when they are gone.