My Reasoning
I've been thinking for quite some time that there should be a way to kill lionesses AND send them to the dynasty, before they are 15 years old. I have been counting down the days until some of my lionesses die, because I need the room but want my precious oldies in my dynasty.

So what if there was an Oasis item that could poison your lioness and cause her to die without completely erasing her from Lioden?

Information
I did some research on poisonous plants that are present in Africa (since there's been a lot of discussion lately about the realism of things like Ligers in Africa) and decided that Oleander is a nice fit to be this poisonous item.

photo credit
This is an Oleander plant. They are toxic when ingested and can cause severe reactions upon external contact as well.

Ideas (feel free to comment with suggestions!)
- Oleander would be an item with one use.
- It would be bought in the Oasis.
- Can only be used on adults. (EDIT: Possible age limit of somewhere between 5-7 years old?)
- Upon use, your lioness would die just as they would have eventually, and would be sent to your dynasty.

"But what about karma?"
Since you paid GB for an item to kill your lioness, perhaps giving you negative karma wouldn't be fair to the players who want to be good. So, I think that karma shouldn't be affected when using this item, to keep it fair for all players of different karma goals.

Possible action text...
"Your lioness came upon a wild flower and ate it, wondering if it was good. Unfortunately, it was an Oleander. The lion has sadly passed away but can still be stored in your dynasty! Go to dynasty?"

(thanks, BritishAce!)

"Hello! I totally support this idea but some people are saying "that is it bad for your king to murder a lioness" maybe when you use it, the message can say:

"Your lioness came upon a wild flower and ate it, seeming it was good. Unfortunately, it was an Oleander. The lion has sadly passed away but can still be stored in your dynasty! Go to dynasty?"

I don't know. The message kind of reminded me of the poisonous plant Hemlock and how some people are it thinking it was another vegetable. But in reality, it was hemlock. Even a single small bite can kill a man"
-BritishAce