An assembly including an autonomous capsule having an anchoring member adapted to penetrate tissue of the heart and an accessory for implantation of the capsule. The accessory includes a steerable catheter with an inner lumen, having at its distal end a tubular protection tip defining a volume for housing the capsule. The accessory also includes a disconnectable attachment mechanism for supporting and guiding the capsule to an implantation site and a sub-catheter housed within the lumen of the steerable catheter, moveable in translation and in rotation relative to the steerable catheter. The sub-catheter and the capsule are movable between a refracted position and a position wherein the capsule is deployed out of the protection tip. The sub-catheter and the capsule are provided with a first fastening mechanism for fastening the two in translation and in mutual rotation, which is disconnectable under a rotation applied to the sub-catheter.