Shakti

When Lord Vishnu was asked by devarishi Narad as to why the Lord was not disturbed by the fact that Laxman had been Gravely injured by Indrajit. at that time Lord Shiva answers the sage by saying that Shesha had the power to imbibe both

1. Halal(poison) and 
2. Amrita(nectar) 

and as such could not be affected by the Shakti spear, but because the shakti spear was given to Indrajit after Pleasing Mata Shakti with his penance, So out of respect for Mata Shakti Laxman accepted the attack of the "shakt spear" on his chest.

https://freeglobaluniversity.blogspot.com/search/label/NectarPoison