Ways of playing PvP nowadays:
Actually, there are several ways of doing it, the difference is just in accessibility from the 2'nd / 3'rd part. The "Lore" way is to use the in-game PvP function which is available in "Custom Quest", however, if you go this way you will be forced to load a mod and you will end up with non-default server map name, hence, only players with the same mod will be able to join, for a player that does not use mods it will show up as an INVALID server, it ruins the taste of PvP.

The solution:
(Make a save of Engine.dll before trying it!)
The solution I could come up with was to give you players slightly improved Engine.dll file which has changes to it so when you host a game / decide to join it will be counted as an actual PvP (If you're the host it will be by all means, however, if you join to a server while having this DLL instead of original, you will need extra tools to turn the joined server into an actual PvP, so it will be the mode of the server's host's choice). The DLL editing is necessary so all players will see our server an an OK instead of INVALID which is promoted by mod-using server creation.

What is actually changed in this DLL?
Ignore if not interested
So, in Engine.dll there is a function named Engine.GAME::GameInfo::SetPvP
using a disassembler I just edited the pointer which was taking the value for the register that refers to the value of PvP part from a register in which was loaded the state of Auto-Party / PvP. Basically I just loaded a constant value without allocating any memory [just rewrote it].
Installation:
In the .rar file I've uploaded there are 2 folders:
"PvP Engine.dll" > Engine.dll
"Original Engine.dll (if you wanna switch back)" > Engine.dll
All you need to do is to put the Engine.dll file into the game's root directory:

*Disk*:\SteamLibrary\steamapps\common\Titan Quest Anniversary Edition