exterminate, destroy, blessing
戩 centers on the act of completely wiping something out, whether by force (exterminate, destroy) or by a positive divine act (blessing). The blessing sense likely extends from the idea of removing evil or misfortune.
戩 combines 晉 (advance, promote) with 戈 (halberd, weapon). The exact historical development is uncertain, but it likely expresses the idea of advancing with a weapon to destroy, or metaphorically advancing blessings.
The left side 晉 suggests advancing or promoting, and the right side 戈 is a weapon. Imagine a weapon advancing to completely destroy an enemy, or a divine weapon advancing to cut away evil and bring blessings.
For セン, picture a scene of destruction where a thousand (千, せん) weapons (戈) advance to exterminate everything: 千 -> セン.