Homebrew: Switch-Axe from Monster Hunter World

[wynlyndd]

Okay my DM gave out this item (well we quested and it was the reward)

It's an Axe (I think 2hd BattleAxe but I'm not sure) with Reach.
When you crit, it gets a charge. It can hold up to 5 charges.
As a bonus action, it turns into a Greatsword and can upon a hit, dismiss any amount of charges for 1d8 additional damage/charge.

I'm at a loss. I'm sure the easier thing to do is have two entries, one in Axe form and one in Greatsword mode, but how do I track charges? and it is not necessary for Hero Lab to necessarily roll the additional dmg when charges are used....but it would be cool.

But again, I am at a loss. Any pointers for me to look at would be appreciated.

(so many homebrew items recently)