Why Indra is called as Paksan (पाकशासन) ?
As mentioned in Vamana Puran – Chapter 71, Narada ask Rishi Pulastya (पुलस्त्य) why Lord Shiva directed Lord Indra to visit Malaya Mountain ?
Rishi Pulastya begins this story :-
Demons by the name of May (मय) & Taar (तार) who were followers of Andhasura (अन्धासुर), they after being defeated by devatas in the war, were eager to evade to Paatal loka.
During their journey towards Paatal loka, they saw a beautiful mountain called as Malaya mountain and decided to make shelter there. When Lord Shiva came to know that asuras are residing on Malaya mountain, he sent Indra there.
After reaching there Indra called asuras for war, asura too came by taking their weapons. Soon Indra held May (मय) and other asura by his arrows, and in the meantime he killed another asura named as Pak (पाक) using his sharp arrows.
तत्र नाम विभुर्लेभे शासनत्वात् शरैर्दृढैः।
पाकशासनतां शक्रः सर्वामरपतिर्विभुः।। 71.14
Since Indra ruled over the asura Pak (पाक) using his strong arrows, therefore the lord of all devata achieved command over Pak (पाकशासन)
Why Indra is called as Purandar (पुरंदर) ?
तथाऽन्यं पुरनामानं बाणासुरसुतं शरैः।
सुपुङ्खैर्दारयामास ततोऽभूत् स पुरंदरः।। 71.15
In the same way using his arrow decorated with feathers, Indra killed another asura named as Pur (पुर) who was son of Banasura (बाणासुर), therefor Indra is known as Purandar (पुरंदरः)