This is my first guide. I am main sup and Pyke is one of my favorite champions, all of the other guides are the same and I don't like the builds on them so i thought of making my own.
This build will really pay off when you master pyke, you will spam your Death from Below and execute enemies non-stop literally. if you successfully execute an enemy the window given to reuse your Death from Below will be even higher than its cooldown (with the help of few cloud drakes). the miss penalty will be so low that you will reuse it in less than 20 seconds if not less depending on your jungler

-pros

• enemies on low health will be executed on sight
• enemies who are not careful will be grabbed on sight
• enemies who are alone will be stunned on sight
• very high mobility, you'll be everywhere
• hit and run tactics are easy and rewarding (beginner friendly to some extent)
• disappear when an attack fails
• when you get good, enemies on low health will tend to go back to base leaving map for your team to roam
• can stop enemy carry and good enemy players if you have at least one good player on your team
• can save your teammates from getting killed (stunning enemy or grabbing them away or executing them if low on health)

-cons

• squishy
• valuable to cc if not taking right items or runes
• need decent adc to dominate and bad adc can make your game hard
• can't carry without good or decent players on your team, if enemy team is fed it's near impossible to win especially if enemy team tanks the turrets to kill you
• teammates will sometimes accuse you of stealing kills

Fleet Footwork- I chose this mainly for sustain in lane, you need to level up as much as fast as you can, staying in lane even when not farming will grant you xp when enemy minions die, staying full health will let you stay there. also it deals good damage on first hit (only one you probably need) and movement speed for escaping.
Presence of Mind - nothing worse than running out of mana when you can execute an enemy except running out of mana when trying to run away. Triumph is also good but I don't think it's as useful.
Legend: Tenacity - most useful, helping against cc will save your life A LOT.
Cut Down not much useful but others are just worse. all other champions have higher health so it's always active.
Taste of Blood- will give you even more sustain and will help you against ambushes
Ultimate Hunter- obvious why

laning phase
general

• the most important thing is DON'T go back to base a lot, you have one of the best sustain the game especially with Fleet Footwork, if your adc goes back to base keep farming on lane (execute minions) so you can level up as fast as possible and finish your build
• don't buy potions it's a waste of money your health is low and will regenerate fast especially with Fleet Footwork
• only use your Phantom Undertow to stun when an enemy is a bit farm from the other
• if your adc get caught, go to the enemy's side, use Bone Skewer to get them away from him then stun with Phantom Undertow
  levels 1-5
  
• keep harassing with your Bone Skewer but watch out for your mana, you need it for escaping ganks and if you get caught.
• DON'T play risky, aggressive yes but not stupid
  after level 6
  now you get your R
  
• again don't be stupid, get enemy to low health before going in, if enemy on high health you will get sandwiched between the sup and adc and you will die before you can use your R
• all enemy players have Flash, wait for them to use it then use your R, even if it means you don't get the kill sometimes
• don't rush your Death from Below early game (late game feel free to spam it).
  late game
  
• when near turret get stay on safe distance and try to grab enemies with Bone Skewer for the rest of your team to kill them
• If your enemy has lifesteal/healer get Executioner's Calling ASAP. high damage can get the enemy out of red.