In short...

In short, when we say ‘ ya thar Allah ’ we mean ‘O one whose blood is the blood that Allah venerated and preferred over the blood of others.’[^2] أَلسَّلاَمُ عَلَيْكَ يَا ثَارَ اللَّهِ Peace be unto you, O the spilled blood, whose avenger is Allah One of the most clear expositions[^3] for thar Allah is that it refers to that blood that has been spilled in falsehood and injustice, and is attributed to the wali al-dam (one who has the right to avenge for the blood).

For example in one of the ziyarat of Imam al-Husayn (AS) we address him as follows: ...وَاَنَّكَ ثَارُ اللهِ فِِي الاَرْضِ وَالدَّمُ الّذِيْ لاَ يُدْرِكُ ثَارَهُ اَحَدٌ مِنْ اَهْلِ الاَرْضِ وَلاَ يُدْرِكُهُ إلاَّ اللهُ وَحْدَهُ ...and that you are the blood of Allah ( thar Allah ) in the earth and the blood that none of the inhabitants of the earth can avenge, and none save Allah alone can avenge it.[^4] This, however, does not contradict those salutational recitals and supplications that encourage us to ask Almighty Allah to enable us avenge the blood of Imam al-Husayn (AS) with our present Imam (AS), for he is a vicegerent of Allah on earth and His medium, and can thus serve as Allah’s representative in avenging the blood of Imam al-Husayn (AS).

The Holy Qur`an says: وَلا تَقْتُلُوا النَّفْسَ الَّتي‏ حَرَّمَ اللَّهُ إِلاَّ بِالْحَقِّ وَمَنْ قُتِلَ مَظْلُوماً فَقَدْ جَعَلْنا لِوَلِيِّهِ سُلْطاناً فَلا يُسْرِفْ فِي الْقَتْلِ إِنَّهُ كانَ مَنْصُوراً Do not kill a soul [whose life] Allah has made inviolable, except with due cause, and whoever is killed wrongfully, We have certainly given his heir an authority.

But let him not commit any excess in killing, for he enjoys the support [of law.] (17:33) Al-Bahrani in his Tafsir al-Burhan , while commenting on the above verse narrates the following tradition: عَنْ مُحَمَّد بنِ سِنَان، عَنْ رَجُلٍ، قَالَ: سَأَلْتُ أَبَا عَبْدِ اللهِ عَنْ قَوْلِهِ تَعَالىَ: وَمَنْ قُتِلَ مَظْلُوماً فَقَدْ جَعَلْنا لِوَلِيِّهِ سُلْطاناً فَلا يُسْرِفْ فِي الْقَتْلِ إِنَّهُ كانَ مَنْصُوراً.قال: ذَلِكَ قَائِمُ آلِ مُحَمَّدٍ، يَخْرُجُ فَيَقْتُلُ بِدَمِ الْحُسَيْنِ...