When Blight is added to a land that already has Blight, cascading occurs: you must add an additional Blight to one adjacent land. If that adjacent land also already has Blight, it cascades again from there, and so forth (page 9, page 15).

Order of Cascading

• You only add Blight to one adjacent land per cascade: When cascading, you choose a single adjacent land to add Blight to, not all adjacent lands.
• If the chosen adjacent land also has Blight, the process repeats from that land, again choosing one adjacent land to cascade into.
• This can create a chain reaction, but at each step, only one adjacent land is chosen for the next cascade.

Chain Reactions

• Yes, cascading can create a chain reaction affecting several lands in one Ravage, but only one path of cascading is followed at each step.
• The process continues until you add Blight to a land that did not already have Blight, or you run out of Blight tokens.

Player Choice

• The players choose which adjacent land receives the next Blight during each step of the cascade (page 11, Option 1 and Option 2 in the example).

Summary

• Cascading only ever adds Blight to one adjacent land per step, not all adjacent lands.
• Chain reactions are possible, but only one chain at a time, based on player choice.
• The order is resolved by the players choosing the path of the cascade at each step.

Source: page 9 (CASCADING), page 11 (RAVAGE EXAMPLES), page 15 (BLIGHT), page 21 (Note: Two Blight rules...)

Source: Core Rulebook (pp. 11, 15, 21, 9)