A build is three systems talking
Your Nightfarer decides what you do. Your relics decide what you walk in with. Your dormant powers decide how that shape changes during the run. A build is those three agreeing, not one of them being excellent alone.
Most weak builds fail because two of the three point in different directions.
Start from the Nightfarer
Pick the class first, then choose relics around it. Building relics first and finding a class for them rarely works, because colour slots and class-specific relics limit what is even possible.
Ask what your Nightfarer already does well, then look for relics that make it cheaper or safer.
Let rarity guide value, not decisions
Higher rarity rolls more effects, which usually means more value on one relic. But three effects that do nothing for your plan lose to one effect you will feel every fight.
Read the effects. Rarity is a hint about size, not about fit.
Stop converting too late
Every build has a point where it stops scaling and starts needing a different direction. Recognising that point saves more runs than any specific upgrade.
If the same thing keeps killing you, changing damage output is usually the wrong answer. Patch the failure, not the fantasy.
Check it against the Nightlord
A build that clears the map beautifully can still lose the final fight. Before committing, ask what the Nightlord asks of you, and whether your build can answer it.
Practise one build until it is boring. Depth beats variety when you are trying to finish expeditions.