Major Artifacts provide a blanket boost to your heroes power level. They are unlocked by completing various mission chains. When you unlock a major artifact for one hero, that same artifact is unlocked for all heroes.
Upgrading major artifacts requires polychoron which you receive by completing faction challenges.
Below is the amount required for each upgrade level.
1: 8 polychoron
2: 10 polychoron
3: 14 polychoron
4: 18 polychoron
5: 23 polychoron
6: 30 polychoron
7: 39 polychoron
8: 50 polychoron
9: 65 polychoron
Total: 257 polychoron