The Shadow Blade replaces your Runeblade once you have maxed out all 6 runes on the Runeblade, which is roughly around level 1050. When this happens, you start over with no runes and no levels, similar to starting at level 1 with the Runeblade. You keep your Rozh Gemstone level and Jaagu Soul Stone level when the Shadow Blade is activated though.
An obsolete scroll by Warlock Markoon Derci documented his experiments on a 'Shadow Blade'. The peaked Runes ultimately became absorbed in the blade itself, permanently altering its characteristics. This in turn made it possible to re-awaken the Runes with a much higher intensity than before. Thus far the possibility of a Shadow Blade transpiring has been considered but a hypothetical theorem.
- There is no known variation of strategy for the Shadow Blade from the normal strategy used with the Runeblade.