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I had noticed that during one of my fights with Yhorm, I did about double the damage I usually did when I managed to hit his arms or an area generally glowing kinda red on his body, though this only got me to deal 60 in a hit rather than 30. My Gamer senses kicked in and said 'glowing areas are usually targets'. might be worth testing out to see if its viable.
the first time I attempted him I didn't bother running for the loot in the back, so I killed him the old fashioned way.. Took about 10 minutes for the entire fight, and I was using a 2hander that did about 100 dmg per hit on his legs, 200 dmg on his arms.
Tvtropes confirms the arms are weakspots, for a given defination of "weak."
forgot to specify, this is in case people want a challenge, like killing him without using Storm Ruler or NPC help
first boss who killed me in Dark Souls 3.

realy hard boss!!!
Am I the only one surprised to see that he, and I guess every giant, has a proper human face? In DS2 they all just had that gaping hole in their head, which was so creepy, but in the Siegward cutscene you see his face!
all his attacks can be blocked
This boss is the second best i've encountered so far for Souls, Embers and Sunlight Medals farming. Once you get Storm Ruler, you can wait outside and just farm with co-op. Usually, people get in with 2 phantoms. If at least two guys have the sword, the battle takes no more than 3-4 minutes, earning:
- 1 Ember
- 1 Sunlight Medal
- 9000 Souls (9900 with Covetous Silver Serpent Ring)
Somethings wrong with mine then. I'm trying to fight him using storm ruler but it only does 34 damage every hit. I think It might be a bug
Special death animation without storm ruler?
died 2 3 times, my long sword had not much effect, so i upgraded a ultra great sword dodge his attacks and hit his head. did that for like 10 min so like evry lucky souls player i deid with him only need to get hit 1 more time to die, yes that was fun. so i did him agian killed him getting only hit once with out storm ruler, yes it felt good but not as good killing the snake lady in dk2 first time playing with the posion.
Make sure you fully load the Storm ruler special attack. It takes around 3-4 seconds, you will see when it's full. Then lock target in Yhorm and use it. Here is a video that shows it:

https://www.youtube.com/watch?v=c2ysVm2mP3s
Storm ruler not working at all. My weapon that has been infused and upgraded to 6 does more damage than this piece of crap, and even with that it'd take 10-20 minutes to kill this guy. Bug(if so, pretty major one) or trolling?
You have to charge it until it lights up, otherwise it doesn't do *****.
You have to charge it up with your weapon skill and than release the charge with either light attack or heavy attack. If you just try hitting him with it, it acts as a regular weapon.
Seriously, watching Sigward pulling out his storm ruler and absolutely kicking Yorm's *** was the most beautiful thing i've ever seen. A fitting end to the questline.