Npc invasions will still occur while offline through scripted events, just find the right spots and youll encounter them naturally. The main ones like magnus the beast claw appear during varre's quest line, specifically near writheblood ruins, while others show up at fixed locations like the broken bridge in altus plateau.

These fights are pretty important since they often give unique rewards and help you progress - for example, beating the right invaders lets you get stuff like the lord of blood's favor, which opens up access to mohgwyn palace. Since theyre ai-controlled, you can learn their patterns and retry as needed without worrying about other players getting involved. Just remember, these encounters are designed to be part of the single-player experience, so take your time and adjust your strategy if youre having trouble with a particular invader.