Let me set the scene. You kill a boss and bring the head back to your base to plant over your mantle like a viking should. As you place it a message appears before you "Five Nights Remain." You hear a rumbling as enemies from that bosses' homeland begin to rush your walls. They are cursing you, calling out for their lord, desperately trying to reach it's head and return it to it's not decaying body. You cannot sleep and cannot rest until the beasts are slain. You fight them off, loosing arrow and swinging axe in defense of your village. Lightning strikes as the last welp falls – Odin is watching. But the damage to your walls is noticeable and your hunger is increasing. A message appears in the sky "4 nights remain." You spend the next day repairing and preparing for the oncoming storm – another night in hell.
The rain begins to fall, the night is growing ever more red. Over the course of 5 Nights the mobs get angrier and more dangerous. The head begins to scream out towards them, cursing you as you fight off the enemies on all sides of you. After a long 4 nights you cant taste victory. And then it appears. A unique monster you've never seen before, guardian of the homeland you've just ravaged. Behind it are lines of attackers rushing towards your walls. Will you survive?
Basically, this would be a horde defense mode triggered by killing and keeping an enemy head within your base. It would add a certain level of prestige to having the trophy within your base and spice up the game without needing to dramatically change the gameplay (unique unit isn't really necessary). What do you think an appropriate amount of days would be and do you think there would need to be a unique reward?