Soul Sickness and other campaign timers

I am planning a pseudo Ravenloft campaign for 3.5. The campaign will feature a fairly short, open world style, main story arc along with a good number of side quests/arcs.

I want to add a mechanic that creates a sense of time pressure while simultaneously giving players enough time to explore the world.

My current idea is that enough time in the Dread Realms can inflict the PCs with a soul sickness that deals Cha drain and, if it reduces Cha to 0 traps the PCs soul in the Dread Realms forever. Good deeds/a special type of item scattered through dungeons can cure, reduce the progress of, or otherwise slow the soul sickness.

However, I have never used a mechanic like this before and appreciate any advice or alternative ideas on how to add limited time pressure to my game.

I would suggest against using Charisma drain as the limiting factor. Charisma-based characters will be penalized far earlier than others, dissuading your players from playing bards and sorcerers and the like. At the same time, the actual time limit will vary significantly from character to character, since they'll have different Charisma scores. If you want to do something like this, I would suggest undispellable negative levels that don't cause permanent level loss (like from wielding a holy weapon as an evil character). The effects on the players' characters will be much more consistent regardless of class, and the time limit imposed will likewise be consistent between each member of the party.

Do you have a plan to introduce your players to the curse? I would suggest some introductory segment of the adventure have some forced time delay or exposure such that the last segment of it is under the first stage of the curse's penalty. That way, the players get to understand the nature of the curse and its consequences organically, and you can teach them about the countermeasures in much the same way. For example, if the players' first activity would be to cleanse a forgotten, cursed monastery, the first day of travel to it you could describe a sense of unease that they feel, with an almost tangible darkness seeping under their skin. As they reach the monastery and fight closer to the center of the curse, they feel worse and worse, until eventually you show them its effects with the first negative level just before the last or second-to-last encounter. The players would have to fight through the last fight or two with that penalty active. When the players perform the cleansing ritual on the monastery, you can describe the darkness coalescing from the air and the monastery's structure and being pulled into the relic they use, but also being drawn out of the players themselves. Afterwards, the relic they used is a blackened, brittle husk, the monastery's darkness is purged, and the players themselves feel better than they have ever since embarking on their adventure... but still with a hint of that darkness scratching at the backs of their minds, trying to make its way in.

For your Ravenloft campaign, consider a soul sickness countdown that decreases each in-game day. If it reaches zero, characters lose Charisma. To combat this, players can do good deeds or find special items to pause, reset, or slow the countdown. Make sure the time allowed is enough for players to explore without feeling rushed, and explain the rules clearly so they can plan their strategy. This will add excitement while giving freedom to explore.