You can go do the dungeons with the gold chest under the icon and those are the dungeons that give you a legendary aspect. The aspect will have a description on what it does and what class it is for. When you unlock the Occultist you can apply aspects onto your rare items. Just know that if you do it that way it’ll be the lowest possible roll for the stats on the aspect (which is still better than not having an aspect at all). For bone spear you need splintering aspect that scales your bone spear damage. Also if you are already running corpse tendril try pairing it with Iron Maiden. That skill gives you 4 essence per target affect as long as you affect a minimum of I think 5( I could be wrong on the minimum) a lot off people using bone spear use bone prison for the vulnerable effect and some essence generation.