The Essence
Al-Haqq is the Absolute Truth. His existence is the only undeniable reality. All false deities, illusions, and deceptions will eventually fade, but Al-Haqq remains forever.
In the Quran
ذَلِكَ بِأَنَّ اللَّهَ هُوَ الْحَقُّ وَأَنَّ مَا يَدْعُونَ مِنْ دُونِهِ هُوَ الْبَاطِلُ
"That is because Allah is the Truth (Al-Haqq), and that which they call upon other than Him is falsehood. (Surah Al-Hajj 22:62)"
In Your Duas
اللَّهُمَّ أَنْتَ الْحَقُّ وَوَعْدُكَ الْحَقُّ
"O Allah, You are the Truth, and Your promise is the truth."
— Sahih al-Bukhari 1120