Useful items

/kill @e
kills everything including you

/give @p stick 1 0 {ench:[{id:19,lvl:96999}]}
Gives you a fancy knockback stick

/give @p blaze_rod{Unbreakable:1,Enchantments:[{id:fire_aspect,lvl:100000}]}
Gives you a stick that sets someone on fire until they die or jump into water

/summon fireball ~ ~3 ~ {ExplosionPower:10,direction:[0.0,0.0,0.]}
Summons a fireball, if you punch it, it flies until it hits a block and explodes. Try not to play with the explosion power, as putting a high power level WILL crash your server.

https://minecraft.tools/en/potion.php
A useful minecraft website that allows you to create custom potions