Got a few questions regarding magical weapons and familiars!

The Undertaker has this death scythe thatcs able to easily cut through almost anything, but he's also somehow able to turn it into a sotoba and hide it in his coat via hammerspace when not in use. I may have to nerf it a bit, but would I have to get rid of the transformation and hammerspace effect or would I be allowed to keep it?

As for the familiar, I thought it would be funny to give him a church grim that resembles a dead dog from canon. Would that be a good idea?