Best I can do is Sun O))) > Earth > Sleep > Bongzilla > Eyehategod > Iron Monkey > Goatsblood > Koreisch > Anaal Nathrakh

Shortened it by two.

Anaal Nathrakh -> Mistress -> Iron Monkey -> Eyehategod -> Melvins -> Earth -> Sunn O)))

Within Temptation to Wormrot?

Within Temptation> Stratovarius> Helloween> Iron Maiden> Motorhead> Discharge> Napalm Death> Wormrot

Fear Factory to Dark Tranquillity

Fear Factory > Sybreed > Meshuggah > Decapitated > Fleshgod Apocalypse > Septicflesh > Ex Deo > Amon Amarth > Dark Tranquillity

Fear Factory > Divine Heresy > Trivium > In Flames > Dark Tranquillity

Let's go ahead from Karl Sanders to Volbeat.

I'll try backwards

Volbeat > Metallica > Exodus > Slayer > Possessed > Obituary > Death > Obscura > Spawn of Possession > Nile > Karl Sanders

I'm pretty bad at this

Deep Purple to Catacombs

Backwards again

Catacombs > Skepticism > Shape Of Despair > Funeral > My Dying Bride > Paradise Lost > Type O Negative > Black Sabbath > Deep Purple

For Volbeat to Karl Sanders:

Volbeat > Danzig > Trouble > Sleep > Om > Karl Sanders

That's...surprisingly short.

I forgot to add Om in there. Still, it is shorter than I would expect.

Extol to Bathory

Extol > Opeth > Enslaved > Bathory

Wow, that was fast ._.

Dude, stop doing your own haha.
Lets do Hail of Bullets to Askival

Hail of Bullets > God Dethroned > Behemoth > Belphegor > Gorgoroth > Burzum > Drudkh > Askival

Try Beyond Twilight to Job For a Cowboy

HoB > God Dethroned > Behemoth > Vesania > Emperor > Borknagar > Ulver > Wolves in the Throne Room > Drukdh > Askival

Beyond Twilight > Anubis Gate > Myrath > Orphaned Land > Melechesh > Nile > Dying Fetus > Aborted > Carcass > The Black Dahlia Murder > Job For a Cowboy

Heaven Shall Burn to Thränenkind

Beyond Twilight > Anubis Gate > Myrath > Orphaned Land > Melechesh > Nile > Hour of Penance > Hideous Divinity > Job for a Cowboy

A bit shorter.

Heaven Shall Burn> The Black Dahlia Murder> Abigail Williams> Emperor> Enslaved> Woods of Ypres> Agalloch> Alcest> Heretoir> Thranenkind

Let's see Skeletonwitch to Wormrot.

Skeletonwitch > Goatwhore > Soilent Green > Brutal Truth > Napalm Death > Wormrot

Zombie Ritual to Rhapsody Of Fire

I'm sure there is a faster way to do this, but I got:

Zombie Ritual > Ghoul > Carcass > God Dethroned > Behemoth > Septicflesh > Rotting Christ > Tiamat > Orphaned Land > Myrath > Symphony X > Rhapsody of Fire.

A little variation, try to start in a band labeled Brutal Death Metal and get your way to a band labeled Power Metal, It's not as easy as it sounds.

You can start on Prostitute Disfigurement if you like.

That's actually one of the easiest paths. Bands like Venom and Celtic Frost are always going to be in the middle.

^ I forgot to add: Without ever stumbling upon a Thrash band.
And yes, it is possible.

From Prostitute Disfigurement to power metal without crossing into thrash. Challenge accepted!

Prostitute Disfigurement > Aborted > Bloodbath > Hypocrisy > Scar Symmetry > Mercenary, who are labelled as Melodic Death/Power metal. Or if that's cheating and we're after a bread and butter PM band, then break off at Hypocrisy > Kalmah > Children Of Bodom > Sinergy.

So that's 6 if we're allowing a partial PM tag, or 7 if we're only allowing it to end at something labeled only "power metal". Probably can cut it down by a few, I reckon.

Well, maybe It was as easy as it sounded. I probably just suck at this game.

Stratovarius to that one black metal band, Ebilgulgul or whatever it's called.

Someone do Silencer to Iron Maiden.

Silencer>Nyktalgia>Burzum>Darkthrone>Celtic Frost>Venom>MotorHead>Saxon>Iron Maiden