Wormwood.

Deuteronomy 29:18
Make sure there is no man or woman, clan or tribe among you today whose heart turns away from the LORD our God to go and worship the gods of those nations. Make sure there is no root among you that bears such poisonous and bitter fruit,

Ruth 1:20
"Do not call me Naomi, " she replied. "Call me Mara, because the Almighty has dealt quite bitterly with me.

Proverbs 5:4
her end is bitter as wormwood, sharp as a double-edged sword.

Jeremiah 9:15
Therefore, this is what the LORD of Hosts, the God of Israel, says: "Behold, I will feed this people wormwood and give them poisoned water to drink.

Jeremiah 23:15
Therefore, this is what the LORD of Hosts says concerning the prophets: "I will feed them wormwood and give them poisoned water to drink, for from the prophets of Jerusalem ungodliness has spread throughout the land."

Lamentations 3:5, 19
He has besieged me and surrounded me with bitterness and hardship. . . .

Amos 5:7
You who turn justice into wormwood and cast righteousness to the ground--

Amos 6:12
"Do horses gallop on the cliffs? Does one plow the sea with oxen? But you have turned justice into poison and the fruit of righteousness into wormwood--

Hebrews 12:15
Be careful that no one falls short of the grace of God, so that no root of bitterness will spring up to cause trouble and defile many.

many.

Exodus 15:23
And when they came to Marah, they could not drink the water there because it was bitter. (That is why it was named Marah.)