[X]The sword Cassiel, which you personally hand-crafted during your imprisonment in the warp to travel with you through your time jumps. If you ever need a weapon, you will find it in your hand, ready to serve.
Summoning weapons through time? Cool. Never seen that one before.
[X]The throwing axe Storm's Tooth, a gift from Russ for your work in stabilizing his legion. This perfectly balanced throwing weapon has been blessed with the same technologies that power the frost weapons of VIth legion, slowing down and freezing those struck.
Always going to take the Russ option. You'd think a throwing axe could see service in melee alongside Cassiel as well, but even if not, a stasis throwing axe from Leman Russ tops a lot of 'cool charts' in my books.
Honestly Khonsu would never be picked, Melee is backed by too much Rule of Cool for a Primarch to not use it. Which is a shame, a Ferrus-made sniper rifle would've been neat.
Is it like second magic in type moon? So are we trapped within a linear timeline or we can interact and jump into a body of same primarch existing in a parallel timeline? Power seems a bit vague to me.
Not too vague. It's a fluid timeline. We jump back into our body from up to two years ago safely, then act during the time we skipped back over with knowledge of what unfolds over that time. So if we take a wild risk to gain information, we can then jump into our body from a while ago and know what the intel endeavour told us.
Theoretically, Groundhog Day is within our means.