Vampiric Creepers are a special unit in Stronghold Legends, and only available to players who are currently playing as the Evil faction.
They are recuited from The Sorceror's Tower, costing 200 gold and 75 honour. They are costly in gold and fairly costly in honour, however their abilities will not leave you disappointed and are well worth these costs. You can find out ways to generate Honour in the game here.
Vampiric Creepers are the only unit in the entire game which have the ability to convert enemy units into your own units.
Note that you cannot convert special units from any faction, and you can only convert Men-at-Arms, Archers and Crossbowmen from the available standard units.
To do this, simply select your Vampiric Creeper and attack the specified units that you wish to convert.
Arrows will have very little effect on Vampiric Creepers, and Spears thrown by Men-at-Arms will only have marginally more. Even Crossbowmen, who normally have a very strong attack will do little damage more than an Archer against Vampiric Creepers.
Archers, Men-at-Arms and Crossbowmen will never fight in melee combat against a Vampiric Creeper, because when they are close enough the Vampiric Creeper will freeze them with an orange glow, where you will see a small blue line slowly counting down. The enemy units will be unable to attack, move or do anything else while they are frozen.
This is the process of converting the enemy units, and once the blue line is fully reduced to the left, the units will turn to your player colour and be under your control as if you had recruited them from the barracks. They will be your units from that point onwards, and if there are any enemy units still around, they will also attack those units immediately after being converted.
Note that Vampiric Creepers are entirely unable to defend themselves while they are converting units and will remain this way until the conversion is completed. Once you have started a conversion, it cannot be stopped unless you disband the Vampiric Creepers.
Other enemy units can however attack them during this time without any risk of being converted themselves (at least until the current conversion is over). If Vampiric Creepers are attacked during this period of time, then they will either be killed and the countdown will stop where it is, giving the enemy full control of their units again, or the countdown will complete and the units will be converted into your army.
Note that only units within the nearby vicinity of the Vampiric Creeper will be converted into your army. Once this process is complete, the Vampiric Creeper may attack nearby enemy units again and begin the process of converting those, but they may only convert a small group at the same time. If you have several Vampiric Creepers working together in a group, then you may wish to scatter them over a larger area, as sending them all into one area does not necessarily increase the number of units they convert.
Only a unit which is immune from Vampiric Creeper attacks can succesfully kill a Vampiric Creeper, unless the units attack the Vampiric Creepers from a ranged position where they cannot be reached themselves. The best way to do this is via an Armoured unit such as a Swordsmen, who will quickly destroy the Vampiric Creeper and save the units from being converted.
White Witches, like most ranged units, have little impact against Creepers. Unlike the smaller Demons, they are not immune to fire, and can be damaged by fire on the map. They can also be frozen by Ice units and damaged by this in a normal way, such as via the Ice Queen.
Vampiric Creepers move very slowly, and have no real way to defend themselves against attack by units they cannot convert. They will try to lash out at these units, but will almost certainly be slaughtered within a few seconds, particularly if they are attacked by an armoured unit who are resistant to their attacks and take in virtually zero damage from them. The entire purpose of Vampiric Creepers is in converting enemy troops, and they cannot attack units beyond this in any meaningful way.
When using Vampiric Creepers it's also important to be aware that they can convert an infinite amount of troops, bypassing even the maximum army size within the game that is usually in place. If you bypass the maximum army size, you will be unable to recruit further units or replace any that are lost at the Barracks or via any of the other Military production buildings, however providing Vampiric Creepers are still alive, you will continue to have the ability to convert enemy units to your army.
Warning: Vampiric Creepers can also climb up enemy walls and attack enemy units on walls and towers directly. It does not need siege equipment to do this and can easily climb up onto a wall, drop down the other side, and then reach units down there or convert units on the wall itself.
If you're planning on building walls to block out Vampiric Creepers, you should keep this in mind. They cannot however climb up Gatehouses or towers. To prevent this it's important you station troops periodically along your wall who are immune to their attacks, and can reach them before they manage to get past your wall or attempt to convert units.
Tip: Unless you have a Dragon nearby to create fire, it is virtually impossible to take out a Vampiric Creeper by ranged means in the game. You must simply rely on melee troops to do this for you. If you're playing against a player who has Vampiric Creepers at their disposal, then it's extremely important to protect your weaker units at all times in order to prevent them from being converted.
You may wish to station Swordsmen or other units on your walls and towers near your Archers or Men-at-Arms to keep them safe. If given a choice of nearby units, i.e. there are several convertable units and units which cannot be converted, the Vampiric Creeper will almost always choose to convert the convertable units, even if it means opening themselves up for attack while doing it.
Because Swordsmen are so slow moving, the shorter the distance between your armoured units and the convertable units, the more time they have to react and the more time they have to prevent any conversions. If you can keep control of this situation, then Vampiric Creepers will no longer have any kind of hold over your Castle or be able to damage you in any way.