Victini is the lucky legendary in the T Dmon Data Pack. You will not find it wandering through ordinary biomes. Instead, it waits at Castle Ruin structures, and you will need to search hard to find one.
Locating a Castle Ruin
1. Explore widely. Structures generate as you load new terrain, so long trips in one direction will uncover more of the world. A boat, horse or elytra speeds this up.
2. If cheats are on, type /locate structure and look through the tab-complete suggestions for a castle ruin entry. If one appears, select it and travel to the coordinates. If none appears, keep exploring by hand.
3. Look for ruined stone walls and towers on the horizon. Climbing high ground helps you scan a larger area.
Finding Victini Once You Arrive
1. Walk the full structure, including the inside, the courtyards and any upper levels. Victini is described as waiting at the ruin, so search the site itself carefully.
2. Light up dark corners so hostile mobs do not interrupt you while you search.
3. Come prepared with a strong team and good catching supplies, as you would for any legendary in Cobblemon.
4. If the ruin seems empty, mark the coordinates and check back later. Then look for another Castle Ruin as well. Legendary creatures are hard to find by design.